Well, for starters this is what I did when I placed my scale kids in tubs many moons ago. Not sure if your setup is a free standing tub or its in a rack.
Freestanding tub:
1. You could use a piece of MDF board (1/2 inch would work) as the base for said tub and route out the size of the flexwatt so that the flexwatt is about 1/4 inch below the tub. That's if you have access to a router. Gives you room to place the probe in there as well with some foil tape. Makes sense? There is no direct contact with the flexwatt, just radiant heat and feel better about wrapping the tub with the item you provided in the link. I also use newspaper, IMHO it's more consistent with temps.
2. For the holes, I used my dremel with a drill bit 1/8 in size and made holes towards the opposite end of where the UTH is, around the groove where the lid would normally go. I made them about 1 inch apart and only made a few at a time so that I can monitor the environment over the course of a week. In the event I made too many I just used my glue gun to seal up a few, its safe and seals up like a rivet around the hole.
3. I never drilled holes on the lid (if its a freestanding tub). I was letting too much humidity out that way. I also used small clamps to secure the lid if this is in fact the way your tub will be setup.
Coincidentally I am in the process of making another base for a new snake I have coming for my QT area. I will take pictures in the event you might be interested.Mike I hope it helps.
My herpstat reads 98.4 and the tub is at 90.0. FYI