I have the same kind of tub that I've been working with in preparation for my russian rat snake. I can say that the tub holds in humidity SUPER well, in my case, a little too well where I had to melt more holes than expected. But for a species that needs high humidity I imagine it would be perfect. It's also super escape proof which I love.
I find for air holes I prefer soldering iron because it keeps things smooth, so yes you can try to 'melt' the rough edges to smooth them down. Just wear a mask and be in a well ventilated room! I recommending testing on some scrap plastic beforehand just to get a feel of it first, you'd be surprised how easily and fast the soldering iron melts through! Alternatively you can try sanding it down just beware you may scratch the plastic which doesn't look the greatest.
As for the amount of holes that really depends on how humid it is where you live and in your house. I'd start with the minimal amount you think you need to test it out and take record of how high the humidity is before adding more holes.
As for the uth and thermostat, here is a diagram of how it should be set up.
For heat pads I'd actually recommend using Ultratherm since they don't get as hot and you don't have to deal with a sticky adhesive backingAs long as you have it regulated by a thermostat it won't melt through plastic. A heat pad alone can be fine if your room ambient is above 75F. If that's not the case then, it is possible to set up a CHE but you'd have too make sure everything is safe and if your tub is high enough, which user @Starscream highlights here: https://ball-pythons.net/forums/show...with-pictures)
Alternatively you can use two heatpads ( a thermostat for each) one for the cool surface temp and one for the hot spot.
Just make sure once you'd finished adjusting your tub to test and run temps for at least a week to make sure everything is stable before moving your snake in.
Edit: Just realized this is for a Suriname boa and not a bp, so I'm not sure what the lowest acceptable temp is for them. The ambient 75F quote was with a bp in mind so if someone needs to confirm or clarify what is an acceptable ambient temp, please chime in.