Alternatively, you can call the lid disc 'Beast Ball Disc', the base disc 'Ultra Metal Disc', the base 'Ultra Metal Base', and the trimmed base 'Ultra Metal Trimmed Base'.
Now, this is definitely one of the most complex balls to make, given the addition of the Beast Ball Trim. However, I believe this is fine, given the fact that the Beast Ball is, in lore, one of the most difficult balls to make in the main world, second only to the Master Ball. As for why the lid doesn't require trim, the reason is twofold. One, three blue apricorns make a Heavy Ball Disc already. Two, the recipe as is already uses yellow apricorns, making the need for Beast Ball Trims, which are just a yellow apricorn again, moot.
As for other uses for Ultra Metal, perhaps you can make a machine that can create an ultra wormhole, or (if you buy into the idea that Eternatus is an Ultra Beast), a locator for Wishing Stars to make into Dynamax Bands. Maybe one for Z-Crystals and/or Sparkling Stones for Z-Rings. There are certainly options to think about.