8x10 Film Holder that doesn't have a light leak wanted

It must be just bad luck, but so far I have not purchased an 8x10 sheet film holder off ebay that doesn't have some sort of defect such as a light leak caused by bowing where the darkslide is inserted.  I'm using a very fast emulsion relative to what many people use (for a very interesting experiment) and trying to avoid as many defects up front as things progress.

Anyone know how to reshape a plastic film holder? I am tempted to glue tensioned steel rails on either side to counteract the bow.

Then I can get this rig operating. What you see is a Speed Graphic with its focal plane shutter grafted onto some 8x10 camera, all mounted on a Saltzman tripod. The shutter arrangement works just fine with all kinds of barrel lenses, even the Aero Ektar for close ups, which easily covers 8x10 when extended a bit.  The camera shown is notoriously wiggly, and so is the tripod head it is mounted upon. One would think this might be a rigid setup, but no it is not, as it oscillates quite wildly and in fact if one intended to design it to oscillate, this is a good way to accomplish it.  Nevertheless it is possible to get a sharp photograph by waiting for things to settle down. The focal plane shutter hardly jiggles the camera and is fine for short exposures. Longer exposures like 1/4 or greater are also possible with care.