Patching a fuel tank on my car.

How can I patch a fuel tank on my car without taking it off or replacing it? The whole is pretty small and I think it was caused by a mechanics shop that punctured it with a jack because I found the leak had a patch on it that I had not put on it. It only leaks when the tank is more than half full. I have tried patching it several times with only temporary results only lasting at the most a few months. The tank is some of type of plastic which is what I think is the problem with getting good adhesion on the patch. Each time I have patched it, I have removed the old patch, cleaned it up with degreaser and thoroughly scrubbed it with soap and water. I have sanded it to rough it up a little to get better adhesion. I have removed the gas cap to prevent any pressure in the tank while the patch is curing. I have tried the epoxy patch the you can buy in a parts store that says it for gas tank repairs. I have tried regular JB Weld and marine JB Weld that says works on gasoline applications. I have even tried a small screw in the hole with some JB Weld acting as a thread filler. The patch stays on, but it starts to leak by after a few weeks or months. Is there anything else I can try to patch it? If I have to replace the tank, how difficult do you think it would be to do myself? It is a 2004 Oldsmobile Alero 6 cylinder.