Have you tried just making one out of (folded in my case) card?
I have an Omega 6 x 6 enlarger and have not long made a neg carrier for my new Horizon panoramic camera, which has long negs on 35mm film. I just place this neg holder between the 6 x 6 metal neg holder from the enlarger and it works fine!
My card is thin, as it goes between an original holder, but using two bits of thicker 'mounting board' should work well and should enable you to make a 6 x 6 neg carrier, just fine.
Terry S