Apologies for having sunk out of sight for so long. After being interrupted by travel, the holidays, sickness, more travel, sickness (and so on...), I showed the insert prototype to a manufacturing engineer on a recent trip to Portland. He said that just to get dies made for injection molding--the means I was proposing--could run about $100,000, and, offhand, another workable manufacturing method didn't come to mind.
It's a fairly simply item--one that would be useful to me and, apparently, others--but the dies would need to be large, hefty items that are intricate to make. I haven't given up completely, but haven't seen the way clear yet. Either I find a way to produce the insert, or find a space for a traditional darkroom. If I do find a way to manufacture this, I'll check in.