Since my Coleman cooler is quite a bit larger than most styrofoam coolers and since I use a smaller flame-shaped chandelier bulb instead of a full size bulb, I have a lot of space in my proof box. I'm sure I could easily fit 25 dough balls for proofing since there is also a ridge about 2" down from the top edge (lid is domed, adding more space) and I have a sheet pan that just fits on that ledge, as does my wooden peel.
Also, DH used a light-weight plastic mount for the light bulb, which could help solve the weight issue for the styrofoam cooler.
Al, I use my proofer for starters, bread doughs, pizza doughs and yogurt making. Not only do I make a lot of bread and pizza, but I also make a lot of yogurt because I use it in place of sour cream (always!) and I also make cheese out of it, so the proofer gets a lot of use!
Hope that helps!