I am new to BP's and I have a question about creating belly heat for the BP. The BP will be housed in a 20L aquarium with a screen cover (possibly partially covered for humidity retention). I plan on using Flexwatt for the heat source operated by a Ranco t-stat (as these are what we use at work and I can get them cheap). I also plan on using a DIY humidifier when/if needed (Aquarium air pump and water filled plastic bottle).

My question is what size of flexwatt should I use?
1: Will 1' of 3" or 4" be sufficient for the hot side?
2: Or should I just use 1' of 11" flexwatt?
3: Or would two 1' sections of 3" or 4" flexwatt (spaced a few inches apart) be better for the hot side?

I live in Iowa so winters get pretty cold and we keep our house relatively cool (high 60's at night and low 70's during the day).

Also would maybe putting a 1' section of 3" or 4" on the cool side just to make sure that temps don't fall too low on low side (controlled by it's own t-stat of course).

Any opinions, help or comments are appreciated. As I want to make it's home the best I can.

Thanks in advance.