Advice for this temporary baby tub setup?

This tub is 12" tall, 12" long, and 6" deep. I am planning on getting a baby in August, so I have plenty of time to fix and monitor humidity/temps! When it gets larger, I plan to move it to my 40 gallon glass tank. I'm gonna go crazy with the clutter when I move it so it doesn't feel overwhelmed.

My questions are:

1. Do I need more holes melted? It currently has 2 rows of holes all around the top, maybe 36 all around.

2. Advice for finding big clips strong enough to hold the non-clipped sides? XL binder clips seem to only stretch 0.8", and the lip measures 2" tall.

3. What heat method/product do you suggest for this small of a tub? I'm scared of it being too small for a nice temp gradient. I have the ability to heat from the top (can cut a hole and cover with whatever material is suggested) or use a heat mat. I see people suggesting to adhere the heat mat to a resistant ceramic tile and place underneath, but not sure if that would penetrate through the bottom of the plastic well due to tile thickness. (Sorry for all the mat questions! First snake!).

I believe I would then lift the tub off the ground with "feet" so there's less of a hazard, and more air flow. I've also seen some people say that heating the plastic that much with a pad (this one is a Hefty) releases fumes that could harm the snake... opinions?
...
I am used to keeping a bearded dragon, so this is very new to me! The fact that you don't need UVB or a crazy hot basking area is so nice. In the end, if this is too small to get a good gradient safely, I will get a larger tub and follow advice for that one. Thank you for reading!

I'm not an expert but is there a reason it can't start in the big tank if it's cluttered up? They start in the wide world on their own so do they need a small tank? I realize baby snakes can hide very easily in a larger enclosure and I remember how new-parent-nervous I was when I got my snake but she almost always pops her head out and says hi when I open her enclosure, unless she's in shed. Occasionally when not in shed but who knows what her sleep schedule is like. They're curious animals who need to know what's going on and will probably feel more relaxed if you're not reaching down like a hawk and body snatching them every other day. Anyway I hope a more experienced person chimes in, and I'm excited for you!

Sent from my SM-N960U1 using Tapatalk

I would add more air holes. I usually do two rows of holes, with each hole being about an inch apart.

Heating can be done with a small heat pad and a thermostat. They don't need much heat, only about 87 on the warm side, so not warm enough to melt plastic or release fumes.

As for the clips, I'm not sure. It's hard to tell exactly how that lid closes. The bins I've used always have the lid overlap the bin and that lip is where it gets clipped.

My advice is don't get overconfident and skip the clips. I had a snake escape out of a lid just like that. They can become very flat.