When purchased, fingerboards are usually too thick and need thinning down, the simplest way is with a very sharp plane but they are difficult to hold steady without damaging them.
A simple method to hold them firmly in place is to cut the same shaped channel in a piece of solid timber, say 300mm x 100mm x 50mm, hollow the bottom of the channel to the same bevel on top of the fingerboard and position the fingerboard in the channel, upside down.
When using fingerboards with the nut already attached, a small extra hollow under the nut area when the fingerboard is firmly in position, will suffice.