I recently assembled my first EC87's using Sergent's assembly fixture.
I also wanted to have some of the short and long versions of these, and I preferred using an assembly fixture instead of, well, whatever.
So, I made a variable assembly fixture.
There are photos in the Photos section under "Sergent variable assembly fixture". There is a photo of it set up with an EC87+130 ready to build, and some assembled couplers above. And there is another shot of the fixture in pieces.
I attached one of Sergent's assembly fixtures to a strip of Evergreen .250 x .500 with four 0-80 flat head screws. I then cut the assembly in half with a razor saw. These two pieces will fit in the base, which is .250 x .500 with a back of .125 x .375 set vertically and glued on.
.040" has to be removed from the gap between the two pieces of the former fixture. The razor saw I used removes .023", so there's a bit of filing to do. Ideally, you take off just enough so that the fixture will accept an EC87-040 coupler when the pieces are jammed together. It probably doesn't really matter, though. You could probably clamp one side, install the other side loose, and the join them with the EC87-040. And then clamp the other piece.
I made some spacers to set up the fixture to do both the EC87+100 and the EC87+130. You can see them in the photo. Again, you probably don't need them, as you could do your setup the same way as with the EC87-040.