I don't like the effect of the 'tea strainer' discs when the outer holes are opened. I would find a lens cap that fits and buy few to make single (center) hole covers of proper sizes. But that's just me.
Yes, that's the way I've gone at it too - thought I might as well post an ad here for discs too, if you don't ask, you don't get.
Surprisingly I found a lid from a plastic gravy pot that fits perfectly over the outside of the barrel. Before any other Imagon 250 in barrel owners get excited about this source of cheap lens caps, it's a fairly obscure brand from South Africa that my wife bought for me on a business trip in Singapore. The gravy was terrible, but at least the pot has found a use. I digress.
After cutting out the inner part it does a good job of holding black paper discs in place at the front of the barrel, close to where the custom built discs would have fitted in place.
It's a bit tricky to find out exactly what size the holes should be for a given H stop - I thought if someone else has a 250mm Imagon it should be possible to just copy the size of the holes in their discs, the pattern of the holes is simple to copy. Anyway, I can make any old pattern that I can cut and see how weird it gets. I read an old post on LF forum where someone suggested making a disc that covers the centre of the aperture (a circular bit of card held in place with some thread) to make the lens go into fuzz overdrive. I've done something similar, it's pretty fuzzy (of course). The focus changes with those discs, so it's fairly difficult to figure out if it's fuzzier than using the empty barrel (difficult without actually making some negatives, rather than than messing around like a second rate origami student, that is).
I also made a disc similar to the original Imagon disc (big hole in the middle surrounded by 12 small holes - small holes made with a paper hole punch) and I also didn't like what it does with highlights.
One problem with making diy discs is that figuring out what H / F stop they provide is a bit tricky. I spent some time earlier relearning how to calculate the area of a circle. I'll probably stare at some figures on a bit of paper before deciding f16 sounds right
