That is an interesting suggestion. Where can one obtain this mylar?
My suggestion is a piece of completely exposed, then developed B&W 4x5 film (T-Max 400 (TMY) would be my first choice), taped at a slight angle emulsion side out to a piece of white, smooth matte paper, such as the back of a piece of priniting paper (Oriental Seagull VC would go with the film, but I use Exhibition Fiber), using black gaffer's tape. Be sure to light the target so that the 7-mil (?) thickness of the film doesn't cast a shadow on the backing paper. Estar is Kodak's name for something very much like Mylar. To keep the film close to the plane of the paper, you can get the gaffer tape close to the edge, but if any fibers stick over the edge, it will confuse the sfr software. You could also use a piece of smooth matte inkjet paper for the backing; if you do that, you can print a Siemens star on it for focusing -- focusing on the slanted edge itself is difficult with an SLR (although easy with the Betterlight scanning back). 120 film would work, too. It's thinner, but it curls more. Developed film curls towards the emulsion side, which is the opposite of what you'd like.
Not developing the film is a possibility. That will reduce the contrast of the edge, but the film will lie flatter, and you probably don't need that much contrast anyway.
As A thinner alternative to photographic film, you might consider the black coated aluminum foil used in studio lighting, or industrial materials such as these:
http://www.tesa.com/industry/electronics/assortment_overview/functional_tapes/light_shading_blockingThe thinner the black material, the less chance of its casting a shadow, but the greater chance you'll bend and crinkle it trying to attach it. Try not to cut your own edge; the cutting equipment used by the material supplier will probably be smoother than anything you can do yourself.
If you have a white backing and a black edge maker, you're going to have a high-contrast target. Keep the exposure down far enough that the demosaicing and other processing doesn't cause clipping, or your sfr program will get confused.
Jim