Mylar over a board.
Don't worry about hot spots. On a light as low power as a fluorescent tube, it's almost impossible to make a hot enough spot to cause issues, you would have to purposefully shape the reflector into a dish, aka make a plant death-ray.
I use mylar on my walls, regular flat mylar, it makes hot spots in my grow room. They tend to fly around the room due to the fans making the mylar flutter, but some remain mostly static. They have never caused an issue, no burns, no issues, if anything those spots have the best bud in the room. I also run more light per sqft than the average grower, so my potential for issues should be higher than most.
Now maybe if you made a DIY reflector for a HPS, then you might have issues with hot spots. A flat reflector is very hard to make significant hot spots with, a curved reflector and higher power bulb is where the potential begins.
