Hi,
Yes for corns prety much any heatmat you can get in the stores would be fine. I imagine back heat would also be fine given the temp gradient you want - but belly heat might give you a more defined gradient. I simply don't know for sure not having tried back heat.
For hatchling corns I used to use the boxes crickets were sold in.
Once they outgrew those I used to use tupperware boxes from poundtretcher.
This year I planned on building a dedicated hatchling rack with slightly more uniform boxes and bought 20 of these.
Imagine my joy in discovering hatchling royal pythons are strong enough to pop the corners.
So I will still be using the lids but will need to make the shelves closer together to prevent escapees.
Not that I think hatchling corns will be able to pop the lids but why take the risk.
dr del