The material in question is commonly called "fingerstock". I measured some out of an 8x10 holder once and IIRC it was .010" steel. Don't get stainless, way to brittle to bend. fingerstock comes in strips with the bottom half solid and the top half cut so there are gaps leaving "fingers". I suppose this is to account for discrepencies in surfaces allowing the fingers and covering material, in our case felt) to better fit against various materials.

I think its primary use is in the electronics industry.

As someone else pointed out you can buy the steel from various suppliers (MSC, McMaster-Carr, Reid Tool and Supply) and make your own.

If you want to get real serious (especially if one considers the cost of ULF film or the time involved in coating a plate) you can use two baffles, one mounted to the septum and the other in a channel above the darkslide slot. Mount one ahead of the other so the end of the darkslide completely clears the first one and allows it to close.

You may read about some people using a neoprene similar to what is used for windshield wipers. Not a good choice. All neoprene will eventually deteriorate, even without exposure to UV. The surface turns to dust and will end up on either the wet plate or the negative.

Oh another good source for all kinds of thin steel of this type is Brownells. they sell gun and gunsmithing accesories and tools.