Your container sounds about the same as mine, and I have a 2.5 foot ball in there right now that is happy as a very fat clam.
You should be fine.
Make sure you put in just enough ventilation holes, though. Your best bet is to start out with too few holes and then add more to help decrease humidity if needed. It's extremely dry where I am, so I don't have too many ventilation holes. As long as you're opening up the lid to check on the snake every day, having only a few holes shouldn't matter.
If you use a pad, I'd go with the 10-20 gal one. Make sure you get a rheostat though, 'cause it'll heat up hotter than you really need it with a tub. And yep, just stick it to the bottom of the container. 
Good luck!
~jenny
-----
"The python has, and I fib no fibs,
318 pairs of ribs.
In stating this I place reliance
On a seance with one who died for science
This figure is sworn to and attested,
He counted them while being digested."
~Ogden Nash
jenny.thegreenes.org
"If you're happy and you know it,
Bomb Iraq!
If you cannot find Osama,
Bomb Iraq!
If the terrorists are frisky,
Pakistan is looking shifty,
North Korea is too risky,
Bomb Iraq!
