So I bought a Polaroid 405 pack holder, got it for relatively cheap because it is missing its dark slide. Figured I could fashion a new one from a spare 4x5 dark slide, but it turns out that that slide might be a wee bit too thick. Should have checked this out before buying, instead of just assuming it could be done, but that's the way I work sometimes.
So I thought I'd ask and see if anyone else here had actually done this, and what material was used.
A long time ago I had the same problem, a dark slide of a 13x18 cm holder broke. No replacement could be found, so I was lucky to find some (rare) black Pretinax, the stuff used to make custom electronic print boards (is this the correct English expression?). The thickness and the flexibility was OK.
At Hobby Shops and well-stocked Hardware stores you can buy small quantities of sheet brass, sheet aluminum and sheet steel. Any of these would be thin enough and make a light-tight darkslide. For the home hobbiest, you could cut these materials "to size" with some nibbling shears (find them at same hardware store), and then file the edges down smooth. No need to paint black, as the metal is lightproof already.
You can order shim stock that can be had in almost any thickness form .001 to .02 inch or more in .001 inch steps and 6" wide. Or Garolite sheets starting at 1/32 inch. Srouce: http://www.mcmaster.com/
Turns out that Micro-Mark has something that seems to have worked--sheets of black plastic of the right thickness that could be easily cut with a knife. Two large sheets for about four bucks. I can use some of the rest of the material to make some pinholes for my Speed Graphic, so the proverbial two birds gotten with one stone. Now I just have get some film packs to try it out!