Yes, my brother-in-law built a solar water heater in Brazil using corrugated panels—it worked great. It was a fun DIY project, but it didn’t last long. If you’re going to build one, it’s best to use materials that will last a long time.

Regarding the diesel heater, if you don’t want to cut another hole, you could buy a diesel heater kit and install it inside, using the existing hole for the exhaust and fuel line. That’s how it’s typically installed in vans and RVs. Keep the portable heater as a backup. Also, for safety, it’s a good idea to upgrade the fuel line and fuel tank.