The next biggest question is on thickness of the rings. The MDF/Fiberglass combination contributes to the strength of the entire enclosure. 3/4" MDF is the standard thickness for single layer speaker rings for fiberglass use. However, you will find them available from 1/8" up to 1 1/2" thick, each has it's place in the mobile electronics field. Flushmount rings, are generally made to 1 1/4" thick total, 5/8" for the mount ring, and 5/8" for the trim ring. For heavy subwoofers (ex. 15" Subs, JL Audio W7, etc), thicker MDF can be used for mounting. JL Audio actually recommends using a 1 1/2" thick baffle to mount the subwoofer to because of it's weight and power. Speaker rings can be used in any thickness as spacers if you need a bit extra space to make your new speakers fit in your doors, or for that extra space behind the subwoofer in it's box.