My kid's Marlin XT-22 came with a single shot adapter. It's just a piece of plastic that fits in the magazine well:
Such a thing could be made with hand tools. Would just require careful fitting until you got it to fit into the mag well.
If I were making one, I would probably first try with MDF just to confirm that I could do it. MDF is easy to work so it would be good practice and proof-of-concept.
If I were to make the final thing out of wood, I'd use maple or maybe bamboo (cheap bamboo cutting boards are a great material). You want something that's dense and hard and stable.
If I were to make the final thing out of plastic, I'd use UHMW polyethylene (the white slippery stuff used for tool jigs and commercial cutting surfaces). You can get the stuff from Lee Valley and home centers; you can also salvage it or something similar from cutting boards. Hard, stable, relatively easy to work.
If I were to make the final thing out of metal, I'd use brass or aluminum if I could find stock of a suitable thickness. Harder to work but might be the longest lived option overall. If I had a lot of time to kill I might try steel, but I think that requires more skill than the softer materials listed above. Could be done with careful filing though.
3D printing might be an option, but I don't like the look of home-printed stuff.
camster might actually have your best option: get a new magazine if you can find one.