the fogger would work just fine imo for your humidity. i would correct that heat first though. that gtp will likely outgrow that tank in the first couple yrs just so you know.
my first steps for humidity would be to close off any huge openings while still leaving a little bit of openings for fresh air flow. after that i'd put that heat pad on the bottom and tape around it real good with cheap foil tape. warming up that soil should increase the humidity pretty good. 72 isn't exactly too bad for a night temp but i'd want it at least 75 myself 78 would be a bit better.
for future reference... never put your heat source bare inside your setup! that's bad jujus.
just curious... where were you getting your information prior to this? i find typically pet stores are the worst source of information. breeders... although more dedicated and typically involved more in the science of things also have their down sides. for instance they feed their snakes every week regardless age and hunger... well except during breeding season. this is not indicative of what they'd find in the wild so usually what we'd see as a very healthy snake is actually bordering if not well over the lines of obese.
don't be afraid to put plenty of heat in there for him. i know everyone says 1/3 of the bottom is a maximum but i find that to be far from true in many cases. it's best to have a decent gradient of acceptable temps than a cold spot that's really out of the ideal range.
anyway since the little guy is arboreal he'll want heat coming from above anyway. you might want to consider making a hood for your tank. i know it's not what you'd want to hear but it's really ideal for him.
i'd think that some plants will do just fine with warm roots. i could be wrong but they do sell uth's at my local hydroponics store. if the one you have won't cut it you might want to see what your local hydro shop has. it might save you some money too.
anyway... since you've got some improvements to make... save yourself some money...
good places to buy... please add to it!