Solution found. I made a collar out of cardboard that goes around the condenser housing. It is spaced to make the condenser assembly ride 1.25 inches higher than it normally does. With my spotmeter, I measure less than 1/4 stop falloff at the edges. That may be optimistic, my meter is less than ideal for this sort of thing. The only downside I can see is that the whole image is about 2 stops less. I can live with that. Now I will poke around and find a piece of metal to make a collar.
Any reason this shouldn't work?