You might also consider just buying a 39mm extension tube and mounting it to a flat lensboard. Nikon used to sell these for their enlarging lenses, but other brands are available, too. Very commonly available and inexpensive.
I never liked Beseler's extended lensboards because the extension is a stamped piece and that means it might not be precisely parallel to the negative/baseboard. The extension tubes are machined and much more likely to be parallel.
By the way, this approach gives you a threaded mount, so you can change lenses without fiddling with retaining rings.