you could always try heating the plants from underneath with a pad instead of heating from above with a bulb.
You’d just need an extension cord run out to your greenhouse to plug it in.
Yeh that works too, but I already have a huge stash of 100 watt bulbs that I bought for various heat uses.
The 3x4 space I have my eye on is next to a retaining wall that has an electrical outlet on it. A regular shop light would probablly work.