The individual IVAR shelves are held in place by four pins that go into holes spaced by about 32 mm. That's one IVAR height unit (HU). A few examples that fit quite nicely:

• Ikea SAMLA boxes (with or without lid): 5 HU
  

• My #ATX power supply conversion for bench usage : 4 HU, but it's pretty bulky and I might rebuild it or use something proper
• DS1054Z: 6 HU
• Rework station: 7 HU

The holes also make it easy to build hinges for temporary side-racks. I have some ideas for that. 

I removed a lot of the stuff that was in there and rearranged the shelves a bit:

From top to bottom:

• 6 HU: Scope
• 5 HU: undecided, might place SAMLA boxes there. Either one large box or two of the smaller ones fit nicely.
• 5 HU: undecided
• 4 HU: Power (ATX PSU)
• 7 HU: Rework station. This doesn't have to be in a good height for working with it since I'll probably take it out and place it on a table for soldering.
• the rest at the bottom: power tools and large hand tools. I'm not sure if I want them in this rack, but I don't know where to place them otherwise.

The X-shaped thing in the background is supposed to make the rack stay upright, but it's also blocking access from the rear. However I think I'll want to be able to access the rack from two sides. It will have to go and be replaced with something else.

At the bottom I Installed a base plate (MDF) and four castors. They don't turn freely though and sometimes get stuck on a mounting screw's head. I might change them for larger ones (will never happen I guess), but this will do for now: