How do the bolts not turn in the base? Is it same as the KH base with sqaure holes and carriage bolts?
By “Appart” do you mean length ways or sideways?
IF LENGTH WAYS:
Just drill 2 new holes in your seat base and use 2 of the old holes(front) and the 2 new holes(back). If the difference is too small then make the two new holes(back) and two old holes(back) into a slot. This may mean that you have to remove the seat cover to tighten the bolts as a sqaure bolt will strip easier in a slot compared to a hole(sqaure hole).
IF SIDEWAYS:
You could make a new set of holes (4 sqaure holes) about a cm or two behind the existing bolts, that way none of the holes should interfer with each other. The position of the holes won’t change anything because you can change the postion of the seat post on the rail adaptor.
If the bolts attach using some other way you could always buy some carriage bolts (KH style bolts) and drill/file some new holes.