For the Omega users who may also search this thread in the future, here's the official Omega D-series under-the-lens 3½x3½-inch filter holder (#429-027). (At least as long as this B&H link is good...)

Omega Under-the-Lens Filter Holder with Red Safety Filter for D5-XL and D2 Enlargers

I have a nice used one of these I got from KHBPhotografix for a heck of a lot less than the listed price in the above link. I use it in place of my VCL4500 variable-contrast head when I need something brighter and must revert to a single-tube V54 lamp. Works well for my purposes using the standard Ilford under-the-lens kit.