aluminum or copper will be better then poly tube, conducts heat better. or in your case cold, put as many coils into the lunch box as possible and get it as close to reservoir as possible so you dont loss coldness in the lines from chiller to tank.
DO NOT move nutes through AL or CU. The nutes are corrosive salts. AL and CU ions, produced by corrosion, are poisons. It is not more expensive these days to go with stainless steel coils over copper, IAC. Copper is super pricey, now.
You can move plain cold water in anything but not the nutes. But, remember what we are trying to do. Not much.
I looked at working fluids and various transfer materials. What was are doing is baby sauce compared to industrial processes that change the heat loads big time.
We are just trying to cool some water 10 - 20 degrees. Not much required. If I was going this way. I would go taller and more narrow.
I would decide what is my main ammo of cold. Is it a gallon jug with some rocks in the bottom so it can sit upright in a container? Good. What container? A small insulated cooler? Fine.
I would take that jug and fill it. Then I would wrap the 1/2 poly.....wait...3/8" in is much better. Wrap the poly in a coil using the jug as the core. Wrap loose so the core can come out. 3/8" makes a fine loop for this purpose. Tape it together with heavy duct tape on the outside to form it into a spaced coil. Make coils so the water in the container can cover them all. (mostly at the bottom of the jug if you don't have a tall cooler.)
Now slip out the used ammo (not frozen) add the rocks,
make a second one and freeze them.
No plumb the cooler. I would not cut into it and ruin the seal. Take a pair of 3/8" elbow. Elbow at the bottom and at the top of the intake line.
Intake comes into the bottom. Very important to push the air up for gas bubbles and the temp graduate works best starting at the cold bottom.
Intake crosses into the cooler, turns down. Runs along the side to the bottom, turns 90* into the bottom of the coil.
Output just goes straight out no elbow at the top of the cooler. No holes in the bottom under high pressure.
So, 2 ammo charges. Just switch when necessary. Keep the cooler topped with enough water to cove the coils when the jug is in place.
BTW< there is no such thing as coolth.
We can only conduct warmth from the water to the ice.
