Flyn, I had the EXACT problem you described when I first changed my transmission fluid. I taught myself a few choice new words that day. However, I read a post in here that if you put some elbow grease behind it, the nut on top of the flat plate will loosen and the cap is much easier to replace. Sure enough, I was timid the first time but then I went at it and broke it loose - the difference was day and night. Replacing the cap has been very easy since then.
DesmoPorsche, this site is not like most - full of egos and competitive. It is truly supportive. Give the maintenance a shot. Most questions are answered either in the maintenance section or, if in doubt, do a search and most topics have been discussed. Howeever, always feel free to ask queestions. The members enjoy helping others.