Is your left hand the 'plucking hand'...on a bass that's strung for a right hander?
If so, I can see the difficulty, although-
You have Jimmy Haslip & Keith Horne to look to. Both, IMO, are/can be monster slappers...and it's the damnest thing to actually see!
On the other hand, if you're fretting with your left hand...it's like what SpidJod said-
Use it to your advantage; you should already have a certain degree of dexterity built up in your fretting hand.
The caveat, though-
Most string players' 'fretting hand' is stronger than the actual 'plucking hand'.
Mike Dimin had an 'exercise' that seems to work: Get a drum stick, lay it into your Thumping hand's palm, use the middle & ring fingers to grasp it...get into thumping position & go back & forth. Helps develope a degree of muscle memory. Good 'practice' in front of the tube, too.
