Hi Andrew,
The reason I don't recommend a piston holding fixture is because a long time ago I bought what turned out to be a bad fixture (I think it was a DDD unit , but am not really sure). In any event, after finding out I don't even need a fixture, I never gave it another thought. In retrospect, the current cox fixture has a 'raised' center. I believe the 'bad one' I had was flat. I've never bothered to see if the cox original fixture works well although I suspect it does. I think the raised center is right & the flat one is wrong...
Even when I slightly 'over-peen' the socket joint, it mostly results in the need for a few minutes break-in rather than a junk part. I use a regular 16 oz carpenters hammer, but just lots of little taps while rotating the tool. It seems to work fine. Lots of little taps is the key. Several different methods work for different people, use what seems to work for you. I can't say my way is best, but my way does give me VERY consistent results & I like that part. Good Luck!
Cheers, Paul Gibeault