Mines plastic too. I tried a styro at first, but it's way too flimsy. So I upgraded it to plastic. If I ever modify it or make a new one I would get Velcro to mount the 'stat, in my haste I just super glued it. Lol. But it works, I drilled a hole so I can fit a screwdriver thru from the outside to adjust it. I'm currently using a 40 watt, but if you mount the stat to pick up the radiant heat like I explained, I don't think it would make much difference except that the bulb would be lit for shorter periods. I will have to test it, but as stable as my temps have been, I'm not worried about it right now.
With your 'stat reading ambient temp there is a lot of lag between when the Bator reaches 99.5 and when the stat senses it, so that's how you get spikes. With a more powerful bulb your spikes could be much worse. I think you're safe with 40 if that's what you're using now, getting the 98-102. I think you will be ok, save the adjustments for after the hatch. This way you can make notes on how it turned out and what you can do better. Experimenting now could cause more problems.
e
