Do you mean get the drum for the roller furler up above the doors that cover the anchor well? My 23 has a pair of stainless tangs that attach to the chain plate in the well, using the same clevis pin that would have attached the lower end of the furler to the chain plate. For some reason, I suppose because of the lengths of stainless strip metal that were available, it is composed of two sets bolted together to make a longer piece. Each strip is about an inch wide, I would guess maybe 1/16 thick, and each of the "half lengths" is composed of 2 strips back-to-back. They are bolted together in the middle to make a longer strip, two bolt holes. The way I install it, the two "layers" straddle the chain plate so it is sandwiched between them, with the clevis and cotter pin holding the strips to the sides of the chain plate. The same approach attaches the upper end of the extender to the bottom of the (Harken, for me) furler. I may be at the boat later and can try to take a photo - I just took it out of the water yesterday so the drum is detached.
I would think the length depends somewhat on your furler. My tang set extends through the slot in between the two closed doors of the well, so the hole for the clevis is about 2, maybe 3 inches above the closed doors. How is yours set up now? Are you adding a furler?