How's it going? Hopefully this will help with a few of your questions. I prefer making as much of my own equipment as possible, just something I enjoy doing. A few years ago when I was ready to purchase an air cooled reflector of some type I first began planning how to make my own. I ended up purchasing a prefabbed cool tube as everytime I would start designing my own, I would second guess the design, not sure if it would be up to par with a prefabbed one or not. So I said screw it and bought one. After receiving it and being able to physically hold it and mess around with it, I realized I had been over thinking and since then have made my own. Anyways, as for how close together the bulbs are I do not have a definitive answer, but I would think as long as proper ventilation were present so as to prevent overheating, it should be fine. Again, I have no proof of this so perhaps someone with more knowledge will chime in. My reasoning is simply that there is no warning on the label or website for an hps or mh bulb pertaining to the distance it must be from other HID lamps. Heat warnings sure, but not distance from additional lamps. Also look up a Son-T Agro bulb. It's an hps with a blue filament inside the same housing. I do not know if the blue portion is technically a metal halide, but it does show additional of some type can be extremely close. Again, take it with a grain of salt and research more first.
As for another option that I personally would feel safe doing without question, would be to simply purchase a second mail box, and when building the reflector leave one end of each off and connect the two together making one big long reflector. Or make two separate reflectors, set them side by side, and attack the edges together, again making one big reflector only this one would be wide as opposed to long and would have separate bulb compartments.
As for the chrome paint I wouldn't suggest it. Something very similar that I would suggest would be to swing by your local auto store and pick up a can of flat white high temp engine paint. Uber durable and flexible, and is designed to withstand temps up to 500 degrees. If you think you need something "stronger", get the brake caliper paint instead, typically good to 1000-1200 degrees. You could also go even higher and get header and exhaust paint, advertised as and considered flame proof, designed to withstand up to 2000 degrees.
As for making it air cooled, take a trip to your local hardware store. I'll use menards as an example. Find the section with home duct work and pick up a pair of what ever size hole flanges you want. You can attach these to the ends of your reflector and can now hook up the appropriate size duct work. As for the wires you could relocate them to come out of the top, side, or travel a short length down your duct work and then exit through a hole you poke in the tubing. You could also reverse the wires and duct work if it suits your set up better and have the ducts attach to the top of the reflectors and the wires coming out the ends. Doing this will provide superior cooling capabilities as heat rises and will want to naturally enter the duct work on it's own, but will sacrifice some amount of lux as there is now a large hole on both ends which used to be reflective material. Whatever works for you.
Also at your local hardware store with be many different size duct work. I use 6" and it was $12 for an 8 foot flexible section, getting cheaper the more you buy. Also in the same section will be multiple sizes of inline fans, typically sold as "boost-a-vent" or something similar. I have a 6" 250cfm in line fan and it cost me $25 brand new from menards. On the shelf below it was also a temperature controlled outlet made by the same company as the fan. It's designed to screw onto duct work (which is how is figures out the temp) and then has temp controls and an outlet on it. Screw it to the outside top of your reflector or vent tube, plug it in, then plug your inline fan into it. Using the thermometer in your grow area you can adjust the temp controls on the controller unit so it turn the fan on and off as needed to keep the perfect temp.
Lastly to seal your reflectors, look in the window section of the hardware store for a piece of glass the correct shape and size. I've had luck specifically looking at basement windows and there are plenty of interesting shaped and what not.
To attack it I suggest picking up a tube of Red RTV High temp gasket maker from the auto store when your there. Then when making the reflector, fold the bottom 1/4" of metal inwards on the two long sides. This makes a flat shelf like support for the glass on both sides. I also suggest using a couple scraps and riveting or otherwise attaching them to both short ends of the reflector and then bending them over to be used as safety tabs just in case
Apply a small bead line of RTV the full length of both edge supports you just made and set the glass in place. Apply a small amount of RTV to your safety end tabs as well and fold up to meet the glass. Use aluminum duct tape to seat and reinforce any edges, areas, or connections that may need it.
And there you go, sealed, air cooled, homemade reflector. Hope this helps and didn't just confuse you!
Oh! I've also had fantastic luck going to Good Will and other second hand shops as well as antique shops and picking up old style candle holders and oil lamps for cheap. Many of these used an open ended glass tube for safety. This tube can then be used to make a cheap and easy cool tube reflector. The last one I made utilizing stuff I had lying around the house and garage, cost me grand total $.25 (for the candle holder) Just FYI!
