There is a backing plate for 4" pads for the XC3401 Flex polisher from the Autopia Store:
https://www.autopia-carcare.com/flex-backing-plate-system.html#.Xm_wi3JKj3g
BUT, you are looking for a backing plate that accepts 3" or 2" pads, and this is kind of a design-conundrum because of inherent gear-drive ring on the plate itself that "limits" the size reduction of the plate to accept the smaller pad diameter.
That said, my idea would be to have a cone-shaped backing plate the reduces down to the size to accept the 3" and 2` diameter pads, keeping the pad area away from the gear-drive ring, almost like an extension rod, but in the shape of a cone. The "problem" or drawback to this design is how it balances the pad area with the pad on it because it is so far from the point of rotational drive. So what looks feasible on paper is probably a bad design in reality and function. (Ah, the joys of engineering design!) In other words, the 3401 was never really designed for smaller pads. NOT that it cannot be done, but then a flat-tipped screw driver is not a pry bar tool, but it doesn`t mean it cannot be used as one (until you break the tip off! Been there, done that!)
My suggestion? Buy a dedicated tool like the Griot`s Garage G8 OR Adam`s new Swirl Killer Mini 12mm LT OR a small rotary like the Flex PE8 Kompact rotary designed specifically for smaller pads. But then it is easy for me to spend other people`s money on detailing equipment!
That said, if you are detailing and polishing motorcycles on a regular basis, a specialized tool for this purpose makes sense (a pun ironically to someone who is "detailing budget challenged", AKA, poor or financially constrained by one`s "significant other"!)
In other words, if you are going to pry things a a lot, you REALLY need to have a dedicated pry bar in your tool box and save your flat-tipped screw driver for slotted screws.