I don't like thumbing over the fingerboard, but over the blank space. Also, I thumb perpendicular to the strings, not parallel, and usually with an open hand. That lets me pop with all four non-thumb fingers in various combinations as well as do percussive slaps on muted strings with the side of my hand with a quick wrist rotation. I think I've only ever seen Flea slap this way, but it seems natural to me, though I can't double-thump from this position, as the thumb never hooks under the strings.
My preference for the 4-string, short scale, reduced-fret, no pickguard, Stingray-style configuration comes about after a long evolution. I live with the limitations, no problem, because it's either this or no slapping. I've sacrificed sonic flexibility to maximize comfort due to some physical issues, but I'd like to recover as much of that sound flexibility as possible by going with an Alembic in my preferred physical configuration. It's unfortunate that I'll have to spend so much money to do this, as I've always liked production basses, such as the Fender American J, just fine. But then I'm happy to be able to rationalize an excuse to splurge on a custom Alembic.
