Dubia heating

Hi my names celestyne and i have a 5 to 6 month old beardie named Nova. I recently got some dubia roaches and im working on getting a colony going. Does any one know of cheap ways to heat up the tank? I living in the north and it gets cold here in the winter. I heard the roaches like the temperature in the 90's to breed and grow fast. But, i know they dont like light and the only means i have to heat them is an old heat lamp. I know it dosent get to 90 in there and my family is getting into bad times, got into a slight car accident, getting nova and making a tank and supplies for her, getting the colony, moms car breaking down, and my birthday in a month (just about everything on my list is stuff i need for nova and the roaches) so finding cheap ways to heat them up so they start breeding and growing fast would be ideal.

I use an under tank heater and house mine in a glass 20g tank. The tank sits on a bakers rack so there is ventilation underneath. I put the cardboard above the heater so their housing area stays pretty warm even in the winter. You can also try using a ceramic heat emitter to heat from above, but I've heard that can dry the air out very quickly so you'd want to keep that in mind.

I have mine in shallow tubs on top of a freezer in my garage. The temps run in the low to mid 90s on the bottom of the tub. You could probably do the same on top of your fridge, I have my bins in cardboard boxes on 3 with 3 sides and open on the top to retain heat and block light as well.