you could use fibreglass, which would be a mess and a pain.
Body filler- just plaster it on in thin coats to avoid air bubbles and sand it down until you are happy. Don't know how durable it would be or how well it would stay on but for $10 it's worth a shot.
Liquid plastic- it exists, I have never used it but I imagine you just make a mold of your boot (bottom part) then pour the liquid plastic in, wait for it to dry and sand any problem areas. They also have liquid rubber but I am not sure how well these products would adhere. If they work the liquid rubber would be awesome as shit, except for when you need to reapply it.
As brought up you might have troubles with shops setting things like your forward pressure or mounting- they might question the safety of your boot and not do it.