You really want something a bit more secure than tape....although I do use really good duct tape over the fuel filler hole, and hold my hand tightly over the tape when I turn the tank upside down to coat the top of the tank.
For the petcock hole, take a flat piece of metal, drill the petcock mounting holes in it, then cut a small piece of rubber or cardboard for a gasket.
Here is one I made for the S65 Honda....which only has 1 screw hole. It is a piece of flat metal bar, a piece of rubber roofing, and a screw. You will need to make one with 2 screw holes.