here are some diagrams
note that when I say something isn’t critical, that’s only saying it’s not critical for accuracy, you may not be able to fit the parts around the router (especially through the whole range of travel)
first the parts, showing different lengths and spacing on the 3-hole arms. Lenths, spacing, thickness, etc is not critical
the 4 main arms must be exactly the same length (hole to hole)
the 3-hole arms must have the holes exactly in line with each other
to drill the holes in the sled, take each of the 3-hole arms, put the center hole on the router bit
(helps if the hole is the same size as a bit you have), rotate the arm and drill through the sled to match the arm.
the holes must match exactly, but the angle isn’t critical for accuracy
finally assemble the linkage
The final critical item is connecting the chains to the center hole of the 3-hole arms, whatever mount you use needs to be exactly the same length on each side from the chain to the hole
the original maslow with the L brackets is not very accurate, but it is very precise (i.e. very repeatable). So if you move the workpiece from side to side under the router, you can have it drill the holes to match exactly (and if you do it in the exact center, the holes will be perfectly aligned vertically).
So you can have it drill the holes for all the arms, and drill a center hole into your backer board.
Then cut out the arms (with whatever saw is handy)
Then drill a hole in the center of the sled and put a bolt/dowel through that hole and the center hole on your backer board.
You can now drill the other two holes for one arm, rotate the sled and drill the two holes for the other arm.
make some spacers to let the arms go past each other, and stand-offs to space the arms out from the sled and you can assemble a working linkage kit.
now, all of this is much more of a hassle than just buying a laser cut kit (either the 45 kit or the top mount kit), but if you are going to DIY a kit, there is not much you really have to be precise on.