Suction cups are terrible on wooden vivariums. I have just removed all sticky tape velcro ect from my viv. I put pics of my viv on here and people weren't happy with tape in with my snake as they could run it and it comes off and sticks to them worse case you could kill you little baby. If your thermostats have a probe you can easily have that in the Viv (drill small hole in wooden case) and then month the actual thermostats outside neatly I used velcro for this. If the have to be internally and permanent (you can remove with glass scraper) then I'd use aquatic safe silicone. It's hard to find I get mine from Amazon. A small blob will hold it even if your snake uses it as something to climb on silicone will hold the weight. If you ever need to remove it a glass scraper with new Stanley blade it will get it off just make sure it's flat and it won't scratch the timber.