I'm no fan of chicken heaters. All too often it leads to burning down the coop! Concentrated heat, dust/feathers, horrible combination.
Your idea has some good aspects. Minimizes dust around the heat source. Feel the tin, it'll be roughly the inside temperature. Swap to the 100w bulb, wait 30 minutes & check it's temp. I suspect it will not be that hot, more like barely warm in 15F weather.
A key thing, watch where the cord goes. Hens will peck it & electrocute themselves. Run it inside PVC if you can. And obviously keep the cord away from water (you would be surprised....).
The key thing is to keep the roost out of a draft, that's the #1 problem. Make sure their water doesn't freeze is also key. You can put your light under their roost, they'll appreciate the rising heat. But don't seal the coop, they still need some air exchange.
Oh, and put a heavy rock on top of it. I can see them knocking it over, rolling it around...
Good luck.