Upload & Sell: On
Alrighty, then. Now I have the answer. Yes it does fit across the diagonal, but as EB-1 said, it's not a very efficient use of space. So, I simply removed two bottom shelves, and the 500/4L IS fits just fine standing up, as shown below. As configured, the cabinet has plenty of room for the three to four dozen lenses that I'll normally be storing in it. It has 37 lenses in it right now. I could easily add another shelf in the top section, and have them lying down, but that probably won't be necessary. I could also add a partial shelf in the bottom section, but I don't think I'll bother.
Within a few hours of getting it set up, the inside humidity dropped from 65% ambient to about 45%. The summer humidity here often gets above 90%, and so I'm looking forward to keeping the lenses a lot drier. Too bad I can't fit in it.
It's a simple system that works with a supply of desiccant material, located in a 'control chamber', inside the cabinet. Every 6 hours, the desiccant material is automatically 'rejuvinated', and the water content in the desiccant is expelled. It's not entirely dust proof, but it has nicely sealed doors, and it's a heck of a lot better than the open bookcase that I previously used.
If I ever get a lens that's larger than my 500/4L IS (can you say 800L?), then I'll remove another shelf in the bottom section. If I can't find space for the rest of the stuff, then I'll just buy another, smaller cabinet to hold the overflow. The new case would cost substantially less than a replacement hood for the 800L.
It's a http://www.xdry.com/dry-cabinet/desiccator-cs-151ss.asp.
I think it's worth every penny. YMMV.