The era of the "cog board" and the ability to build nearly any possible tooth combination has been over for decades. Between proprietary spacing and the matching of the ramps and shaped teeth needed for indexing, manufacturers limit what they offer to the most popular combinations. You can do a mix and match of 7 and 8-speed cogs (use the 7-speed spacers) within reason but it will necessitate buying two or more cassettes or choosing from the few individual cogs still available.