My trip to walmat consisted of a 30gln Sterilite and a $19 heating pad in the icy hot/band aids section. Now it is gunna be hard to find one that stays on since most have the auto shut off at 2hrs. But there is some I couple people bought theres at Walmart but they dont carry it anymore in the stores it seems like. New safety thing i guess. The other way to go is an UTH reptile heater a 10-18gallon one is the one to get and it shouldn't melt the plastic basically nothing over 15-16watts you shoud be fine but here is more info.
This is from another Dubia topic
viewtopic.php?f=76&t=93177&hilit=
"I always recommend human heating pads, as they are safer and can be self adjusted (depending on model). They can be had for around $15 at Amazon.com. Mind you, it must remain on 24/7, so those with an auto shutoff feature are no good.
I couldn't tell you specific brands of reptile UTH's, but anything putting out less than 15watts of heat in a 10" x 12" surface area should be safe for a plastic tote thats 20 gallons or larger in size."