In 2004 I got a TR6 for a daily driver with a spot very much like that. I did what Got_All_4 says: removed the lower & A-post bolts, held the lower part out with a piece of wood, cleaned out old leaves/crud, painted all surfaces with POR15, patched the hole from the back with fiberglass mat then reassembled. Some spray undercoating made it look good as new. Several years later I removed the fender to extend the scuttle drain out the bottom and the repair was still fine.
Jeff