Can you use a smaller prefold, or a plain flat folded 'trim style'? I'd consider just using a lay in soaker pad or a well fitting 'fitted' dipe or something like that just to be able to use it!
I really like trim fitting covers/dipes. Play around with it, after all that work to make it, you should be able to get a few wears out of it!

I haven't made a butt sweater yet, just using the New Conceptions Cover wraps pattern with side snaps. I made a newborn size (seems big, like it's designed to go over a prefold) and the baby won't be here until November so I can't test it yet...
I'm going to play around with the Chloetoes and make a few covers using her pattern, as they are super trim.
Maybe just size it up for the next one you make (any of the sweater left?) and then it will be ok for your prefolds. Those prefolds can be so bulky for when baby is smaller! I always worry the baby is gonna get a back problem from laying on such a big 'bubble' butt!

Hope you get some use out of it!