Building a Rack system and I have a few questions

I'm going to start building a rack system because the cage I have sucks for humidity among other things. I was planning on going with the sterilite 41 qt under bed tub that is about 17 by 34 by 6. Good Size? Rack will have ability to put smaller tubs if necessary.
First question- do I make the rack tight enough I don't need a lid- sounds risky. Or just leave the lid on it and make sure I vent it enough.
Second question. I am planning on flex watt for uth. Should I router a small pocket for the flex watt to sit in so the tub isn't directly sitting on it, I know that stuff is thin. Will the flex watt keep the tubs themselves warm enough, the room is usually around 75 to 80.
Any other suggestions would be great.

With or with out lids you will need ventilation and top ventilation is very inefficient. There are pros to no lids and pros to with lids with is faster to clean to be sure. And a much bigger pain for everything else.

I small recess for the flexwatt is not a bad idea I think you will find a sander better as it is less than 1/16 thick. I would router a place for the t-stat probe however.

I would also suggest that you keep the room at 80 cooler is hard to keep the temps correct the ambient air temp is almost a;ways the room temp give or take a degree or two. There are a lot of people whom have had issues with keeping warm side temps in rooms around 75º not even talking about cool side temps.

Ideally flexwatt is not run over 100ºF for very long, that limits the room temps a lot.

What is harder to control with lids? Ventilation and Humidity? I just don't really want to take the chance of no lids but it it's harder to control I may have to I guess.
I have a cnc router that I will be cutting all the panels out on and will do the pocket for flex watt at the same time. I can control the depth very accurately.
As far as ambient room temp I was also thinking of adding a front door to the rack to hold the heat in. May have to add vents to the rack itself I would assume.
I was thinking about a rheostat control for the flexwatt for constant warmth at the right temp but I also already have two of the hyrofarm thermostats I could use.

Ventilation is hard to pin down. It is commonly accepted that too little can cause serious health issues. The trouble is there is no exact check for too little. Humidity again coupled with too little ventilation is known to cause issues, and too little bad sheds and in extreme cases dehydration.

I have never sweated humidity overly as far as that goes half the hygrometers used are only accurate within 20% RH anyway. (+/- 10%) some are worse. In the words of the Barkers of VPI, err on the side of dry. more ventilation is better than too little. Personally I use double rows of 1/4 inch holes front and back about 1-2 inches from the top. My rack I have a lidless system which is more common. The only issue is cleaning means the tub and upper surface of the slot. Full cleaning I do the whole cavity. The lid users often just need to do the tub and lid and rarely do the cavity.

The easiest way to control the humidity is just add a humidifier to the room. Racks tend to need space heaters and at that point a humidifier is not really much more.

All of the racks I've built so far are meant to be used with lids on. Now the pros are that you never have to worry about an escapee and humidity is very easy to keep up. The negatives are cleaning isn't as fast and feeding can be interesting. On feeding day some of my little ones get so excited the minute I lift the lid off I have a little worm in my face ready for a meal. Overall though knowing I will not lose a snake because of a rack failure is such a positive for me I don't mind the minor negatives.

I would think cleaning with a lid to be wayyy faster! The lid can be moved to a sink washed and disinfected and returned washing and disinfecting the body of a rack is hard.