The Canon MP-E 65 is a true macro -- in that it does not focus at infinity
http://learn.usa.canon.com/resources/articles/2013/mpe65mm_lepp.shtml
"As the magnification is increased, the working distance between the lens and the subject is reduced, from 5 inches at 1x to 2 inches at 5x."
" As the photographer chooses higher magnification, the lens is lengthened from about 3-7/8 inches at 1x to nine inches at the maximum extension, 5x."
Alignment of the camera and subject is critical -- so I would suggest looking into a system that uses a mono-rail. And for really critical focus, nothing beats having micrometer control of both stages. If you're really serious, I'd suggest considering a Sinar P outfit -- but with your camera and lens mounted on the rear standard and your subject placed on what would be the lens 'standard'. Here's a link to a modification which shows how to mount a camera to the rear standard without using the Sinar adapter (which can be hard to find)
https://www.photigy.com/the-dslr-to-large-medium-format-diy-build-nikon-d800e-on-sinar-p-camera/
Of course when you use a heavy outfit like the P pointing down, you need a sturdy tripod. Sinar did make a clamping stand set, which clamps onto a desk but like most things Sinar, it can be pricey.
http://glennview.com/jpgs/sinar/tripod/clampunit/norma/small_4.jpg
I use the P (with additional rails and standards) to digitize negatives and transparencies using a Zig-Align mirror system to get everything plano-parallel. I've also used the Sinar clamp set to make a vertical copy stand for digitizing prints (using a vacuum easel).
You may be able to 'jerry-rig' something -- but if you're serious, it's worth the additional cost of doing it correctly from the beginning thereby making the whole venture far less frustrating.