West Marine or other marine supply store. They have it in a couple thicknesses and sizes. While they come preglued I used 3M spray adhesive
and it still holds 8 years later. Reduces engine heat into the coach also.
PS: I applied mine on the engine side not the cockpit side of the doghouse.
Jan and Rocky
Volunteers for USFWS. 29 refuges with 9300 hrs ea since 2006. 2004 Allegro 30DA, Workhorse 8.1, Banks, 2012 Jeep Liberty Jet, Blue Ox Aladdin, Brake Buddy Advantage Select, 300 watts solar, 5 Optima group 31 AGM's, 2000watt Ames PSW inverter.