Example Registers for 2" x 12" Duct
Yes, the Shormaker Co. makes several styles of custom floor grilles/registers. 

https://www.shoemakermfg.com/Product-Finder/Line/Residential/Type/Register

I’d contact them and they can tell you how to measure for correct fit. They only have a few finishes, but the quality is great (but not cheap).  Let them know if you expect a heavy load on the registers.