Crude option 1:
Dollar store eyeglasses +1 (diopter) is 1 meter f.l.
A pair of the lenses concave to concave will give you a little more than 2 diopter and a little less than 500 mm f.l. (changes due to spacing of elements).
You'll need to fool around a bit to get them parallel. I would do that with no, or a large aperture, so you can see coma in particular. View a light bulb on some makeshift ground glass straight on, or project onto a white surface, wall, etc. That is the effect that looks like comet. Do the best you can to minimize that as it's probably the nastiest aberration (asuming you don't want it). Once that's minimized, put a smaller aperture ...hmmm, probably better to be in front of your pair of lens elements. You can meaure the f.l. approximately as the distance from the center of the lens pair to the wall/groundglass. At this point probably better to focus the outdoors so you're closer to viewing infinity.
Less Crude Option 2: If you can find a pair of close-up lenses, like a pair of +1's - they are already flat and easier to space close to parallel & centered.
Much less crude option 3: Surplus Shed (.com) has a 600 mm air spaced achromat in a barrel for US$11.50. Flat rate shipping is $5, so you may want to shop & buy some other treasure.
I bought one, took it apart, forgot what order they were in, (probably doesn't matter much), never tried it, put it away somewhere, and talked someone else into trying one (never heard from him either).
Maybe you can be the one to follow through.
Creative option #4, similar to something you mentioned eariler. I got a 15" process lens that doesn't fit easily into a shutter, so it's 'unworthy' in the serious ULF crowd. $15 or $20 IIRC on eBay. The rear cell WILL fit in an Ilex #3 (electronic) shutter. Because process lenses are frequently symmetrical, it's probably roughly a 30" f.l. lens that needs a small aperture. There are a number of lenses that aren't as desirable due to the impracticality of mounting in a 'legitimate' shutter.
Also, look up Andreas Feininger's work, maybe Eastman House Museum has his work. I believe he did some ULF using a magnifying glass.
Professor (first name forgotten) Wang in South Carolina (Sandy King knows him) has used or at least recommended the dollar store eyeglass method - that's who told me about it. He has better credentials than I.
Murray