I thought about that also, wanting especially low Eb and D. It's not just the carving of a block. Think about it, especially if the low E of your bass is set up well, and the risk of fouling that up: redoing the nut, carving the block to fit exactly parallel with your fingerboard, for some, drilling or modifying the scroll for secure placement, redoing the stringing, doing the exponential math to figure out from the instrument's exact scale length for placement of stops, working with ebony for the fingerboard part of the extension, which is very hard, can blunt ordinary woodworking tools, and the cost of a E/C extension string, and the time, time, time it takes to do it, with the bass being down for a significant period of this time in order to do the fine fitting.
Guys - have I forgotten anything?
Anyway, I decided I didn't need an extension that badly, because a good one would be at a cost approaching what I already have in my cheap CCB. It would be more cost effective to purchase another bass with an extension fit at time of purchase.