Patrick, I think stpats used to carry the smaller lids, but I don't see
them anymore on their site. MoreBeer has one that might fit you tank:
https://www.morebeer.com/product.html?product_id=19635 ...you'll
probably want to call them about the specific size. I'm not affiliated
w/ any of these vendors...

If you're not going to use gas to blanket the headspace, then you
probably won't be able to use that tank. An option is to make more
than will fit into the tank, and put excess in glass...adding in the
glass portions when the wine volume is reduced after racking. Of
course, this assumes that you have a tight seal on the lid, or have no
lid at all (i.e. using a traditional fixed-volume tank).

Good Luck, -David