I am wondering who among the 8x10 Elwood enlarger users here still has the glass diffusion plate in the lamp head, and can confirm my feeling it's placement would be closest to the negative stage. I also suspect the ones that used sheets of ordinary glass for heat absorption placed it above the diffuser.
If you REALLY want to be helpful, a photo or your best description of the approximate diameter of the most heavily blasted/ground region of the diffuser would give me an idea what it looked like (how Elwood accomplished a solution). I've read the Elwood 1929 patent, but think they made a variety of reflectors over the years (parabolic, elliptical and one other, I forgot). They may have made changes over the years too...
I just picked one up. I may try a sheet of opal glass above the negative stage, but am not opposed to trying my own grinding. I have equipment that would allow me to objectively measure luminance on the projection surface, and a densitometer. I know Jim Jones here just stacked sheets of hand ground glass until he was satisfied with the uniformity of illumination he could achieve.