Don--it needs to be small enough to keep the fixtures from extending out too far. I have an original 36B with just such a fixture above that plate, and I wanted one on another 36B that I was restoring, so I made my own! I used a junction box from a Wayne pump which already has 2--1/2 inch pipe fittings cast into it. I just bored 2 more openings into the round box, at 90- degree angles to the fittings already there. Then I took a 1/2 inch smooth pipe coupling and cut it in half to use as the new fittings. I then welded them to the junction box and there it was! I now had a small junction box with 4 --1/2 inch pipe fittings on it!
From there it was simply a matter of using some close nipples and ceramic sockets , and it was together, and still allowed ample room for a couple small light bulbs inside the ad glass top.
Sorry, I don't have any photos----
