Sheet metal covering holding tanks

Apparently the previous owner of my 2007 Safari 27FB SE needed accesss to the holding tanks. I've got some cut outs in the sheet metal that protects the tanks, that he tried covering up with good old duct tape. I would like to make the proper repair. I was told not to weld a patch because it would be too hot for the plastic tanks. This same person recommended that I rivet the patches on. This seems fine to me but as we all know rivets require predrilled holes. So my question is, how much clearance is there between the sheet metal and the tanks? I do not want to polk holes in the tanks. Thanks