Jeff,
My mast had been un-stepped for about 3 years when I bought the boat, so that part of the job had been done for me. It was just a matter of removing the old step, measuring and trimming the mast base, matching the G10 thickness accordingly, and proceeding from there. I’ve also used G10 for seacock backing, but as you said, it is a bit pricey. I’ve also made my own fiberglass sheets for backing plates with polyester resin and layers 1708 glass. As Pete said, phenolic will also work well, is cheaper than G10, and less labor involved than making fiberglass sheets. I try to avoid wood as much as possible, for fear of rot.