Wow, that is one thick ring. I have no idea what rings go for ($$), I have some extra 3/4in MDF and could cut them on a automated router table (would be perfect circles). Let me know if your interested. Do you need those exact measurements or does the inside circle need to be 4-7/8in?
I had to use 1.5" plus the provided spacer plus the stock speaker cage to get my Aura MR62's into an 87 accord.... if I had MDF sitting around right now I'd do it... I JUST got my router and circle jigs //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif
They need to be that thick because inside my doors there is a metal post that is part of the door where the components will be mounted. I need the inside hole (for the component) to be at least 4 5/8" because thats how big the mounting cutout is. I figure with 5 3/4" the speaker will sit in the ring nicely and still have the screw holes on the MDF to where they won't split the wood.