It's round with flat sides like the top of your cone, with a hole in the middle for the lens. The real Omega ones are two pieces but you could make one of flat aluminum in one piece. Thin masonite or similar might work too, but you would likely need to thin the edges. The Omega boards are usually threaded, if there are no threads, then you would need a lock ring. The diameter for your El-Nikkor is 39 mm.
i use black matboard and black "scratch" board for my flat lensboards (omega).
i cut a hole a tiny bit smaller than the thread size and "screw" the lens into the board.
works ok...
if you need a quick fix until you get a REAL board, this might help ...
Just a little clarification. The D2V uses three different lens mounts/cones to allow various focal length lens to focus properly. Your "cone" was made for 135mm lenses, a longer cone for 150mm and the flat mount for 50 - 105mm lenses. These boards/cones require lens discs to which the lens is attached. Some discs were threaded but most were not requiring a locking ring to attach the lens. Bet your 135mm is attached to the disc with this locking ring system. One word of caution - the lens discs were made with various size openings to accommodate different lenses. Your 50mm Nikkor probably has a 39mm thread. The eBay item mentioned is exactly what you need IF the disc shown has the appropriate size hole. Of course if your make your own you eliminate all those concerns.
Pay attention to Rick's advice; he's right on. Also, you can also try http://www.classic-enlargers.com/, otherwise known as Harry's Classic Enlargers. He specializes in supporting Omega enlargers, especially D2/II's. You may be able to buy the cones and discs you need there, and you most certainly can get any info you need on Omega D's from Harry.