Insulation for 1.75" gap.

Have a 1.75" gap in my walls that will need to be insulated. Trying to decide how I should go about doing it. Its going to cost a good amount of $$$ to layer foam board insulation in a camper this size (7x10, 7'tall).
or t n ttt for short (tnttt.com)
https://www.tnttt.com/