WOW, a question just for ME????? Sweet... Man I feel special today.
Yes, the tub I have does have grating on the bottom... but I'm using a heat mat from t-rex, so its not supposed to attach to the bottom. I have to make sure the mat is positioned properly each time I slide the cage out, but it heats just fine. Could you rig something so that the heat tape attaches to something the tub sits on, instead of the tub itself? It actually heats quite well, as the mat itself reaches a max temp of 100 degrees. I have an indoor/outdoor thermometer installed within the cage, and the outdoor probe is attached directly to the bottom of the inside of the tub near her favorite hide. This way I'm able to watch the max and min temps of the surface that she's laying on, and adjust my thermostat accordingly. For instance, when the house started getting warmer (its springtime after all), the temps on the high side got a little warmer than I wanted, and I was able to recognize it and make adjustments . If I were to do it over again I think I'd get a rheostat instead of a thermostat, just to get less variation in temps.
I have 6 holes drilled on the long sides, and 3 on the short sides. About a week after I set it up, my humidity was still high, so I also have 4 holes drilled on each side of the lid. I found that the amount of water in the mulch regulates the humidity more than anything I did with the tub, so I just leave the lid open whenever I'm in my office the first few days until the appropriate humidity is reached. I generally change the mulch before it ever dries out completely. You may have to adjust your own setup as well, depending on what you use for substrate.
The tub has actually been quite nice. Its sturdy, locks, and while its larger than what was recommended to me for a baby, mine has done superb. I think the hides make all the difference. She's almost 9 months now and if I'd had anything smaller I would have had to upgrade already. I'm hoping I can keep this setup for another six months... but we shall see.
If you have any more questions, feel free to ask... I'll keep an eye out.