I've been researching a build for an 8-string bass lately. All I can come up with right now is using an import 5-string bass kit as a base (sorry) and a Chinese knock-off bridge off of eBay. Plug the holes in the headstock and drill for the repositioned tuners, replace the nut with a blank and cut my own slots.
Or just suck it up and buy the one from ESP (not a fan of the Schecter's body shape). Just not too enamored of spending $700+ for what's probably going to be a one-trick pony in my collection, which means any build I do would have to come in at way less than that to make any sense.