Picatinny rails

Here is a selection of picatinny rails lengths from 40mm - 200mm stepping up in 20mm increments, you should be able to find one that suits the length you need.
There are also 3 different end styles
The measurements are to the screw mounting points, these were designed to be used for nerf blasters and so use the same m2.8 toy screws, m3 screws may also fit