Get a sheet of similar thickness sheet metal, and just rivet it on. Apply a bead of seam sealer around the edges so it won't leak.
The key I've found to making patches look "right" is to make people think it's something other then a patch. Sometimes just painting it body-color works, other times you have to get a bit creative.
For example, get a sheet of decorative radiator covering mesh at Lowes/Home Depot. Paint the solid body patch panel black, and paint the mesh you get the body color your bus will be. Rivet them on together. The black background will make it /look/ like a vent - something that you'll find on the side of an RV/bus. If it looks like it belongs there, people won't look twice - or ever notice that the "vent" doesn't actually go anywhere/do anything.
Find something to put on the patch panel so it looks intentional - a flag, an army logo, maybe get one of those map decals of the US where you fill in each state as you visit.