If you can get the slip-on series lens attachment with the oddball filter threads that fits your lens (sometimes you have to tweak the "fingers" with pliers for a good fit) - but you can't find the glass you need to go in the holder... I did this. I found a step-up ring, in some cases like 28mm to 52mm. You need a ring you can force into the oddball thread on the lens attachment. You can buy a set or if you have a camera shop that stocks stepups new or used, go play around. I measured my series-adapter and got rings that seemed close and found a good fit.
So force the step-up ring into the adapter threads with some loctite (or if the fit is loose, use JB Weld). You have now destroyed the camera adapter, but you can now stick any 52mm filter, lens cap, or hood on your camera. (Or a different modern-sized filter that's close in size.) I chose 52 since even very high-end filters are widely available new and used.
Kind of funny to go shoot a brownie or Holga and think "A grad would be killer on this sky" or really fine-tune the exposure on toy cameras with the correct ND. But it's possibly a good solution to avoid hunting for ancient series filters in good shape, since you'll rarely find the perfect grad or ND you're looking for.