Setting up enclosure - Substrate and gap (potential snake escape) questions

[Chaos-n-Dissonance]

So I'm going with coconut husk substrate. In the final planning stages of my builds, pretty confident everything is going to be amazing. But since I'm probably not going to get the snakes til next month (By the time everything gets here and I have time to put it together I will have missed the reptile show in my area) and there's 1 small thing I'm worried about I wanted to set up the enclosures with everything to test and make sure there are no issues. I'd leave the substrate dry, since I already know my heating element won't cause a lot of drying (using radiant heat panels). Would just be setting everything up to make sure everything works (to check if any of my new hardware is defective) and that the RHP I'm ordering will bring the heat up enough.

2 questions I have for this setup process:

1. If I put substrate in the enclosure with no animal and no added moisture, then let it sit for 2-4 weeks... Would I need to change the substrate before adding in the snake? I can do this dry run without the substrate, but figured since that's gonna put everything 2-3" closer to the heat source it'd be best to do it with the substrate.
2. I'm using these tracks for my sliding glass doors. It says there's 9/16ths of an inch between the two rails, and the glass is 1/4" thick, meaning there's going to be a 5/16" gap between the two glass panes (assuming I know how2math, figure if each rail is the center of each pane then they'd each protrude 1/8" towards the center, 9/16"-(1/8"*2) = 5/16"). Pretty sure a baby ball python could escape with a 5/16" gap (Correct me if I'm wrong, would save me a huge headache). My plan to close this gap was to use some ~1/4" thick foam tape. Would that work? Or anyone have any better suggestions? Really liking the whole sliding door on wheels concept but that gap is a killer. The overlap for the glass will be 2" if that changes anything

[Cheesenugget]

I don't know about the tracks but I would not use any substrate for the first 2 weeks. The first weeks is the quarantine period, even if you don't have other reptiles in the house. Paper towels are much easier to spot mites (commonly picked up from expos) and fecal check (making sure it poops and that its poop looks normal). Coco husk and other substrate can make this difficult for you to see, even with lighting.