Thanks for the response. I've considered using a belt sander to shave ~0.020" off each side of the Canham holders. My main concern is dust. I'm worried that a bunch of plastic dust particles will get in the light trap, be impossible to clean out and will lay in wait only to jump out and ruin my best shots. I may try it with one of the holders to see how it works. I could place some tape over the ends and around the edges of the darkslide to seal out most of the dust.
Before attempting modification of the Canham holders, I would like to know how yours measure up compared to your Lotus holders. The 0.040" number I posted was a typical number. The actual differences measured between 0.032" and 0.056"
Also, have you noticed that the actual image area on the Lotus holders is about 1/4" shifted to the right, compared to the Canhams? If so, do you just allow a little extra room at the edge of the frame when you compose? It just seems a little weird to me that Lotus would make their holders almost, but not quite the same as the Canham holders.