I went down this route a few years ago with a friend and colleague's BMPC4KEF and the optical cell from a Speedbooster Ultra of 0.71x at the same time as I was offering it up to the "big" URSA. The correct position of the Speedbooster optical cell for the image to be distortion-free and free of soft corners, requires the flange face of the mount to move rearwards about 4mm.
You can observe how far back inside the generous workspace of the URSA turret the Nikon flange face had to be repositioned for the Speedbooster/Nikon lens combination to be optically correct.
- URSA 4KVA SPEEDBOOSTER1 RESIZED.jpg (139.73 KiB) Viewed 5829 times
That is not possible with your camera as there is cast bodywork and electronic lens control fittings in the way. You can cheat a little with the position of the 0.71 optical cell but it will not be satisfactory except in the instances where the lens on front will focus past infinity. You may still encounter side-edge and corner softness with wide-angle lenses.
I tried with Nikon F-Mount lenses. The F-Mount flange face is 2.5mm further forward and there still is not enough workspace for the lens to go back far enough. The tail structure of the lenses was in interference with castwork in the throat of the camera.
In your experiments, protect the EF-Mount lens control pins with a strip of insulating tape. If you short those, the camera electronics may lay down permanently. If the little return spring for the release button flies out and goes mulga, there is a fix if you cannot get a new part.
The flint spring out of a Bic cigarette lighter is a fair substitute. It has to be shortened with a Dremel cut-off wheel and the diameter has to be wound in just a whisker by gripping both ends with good pliers and twisting it up tight around a small jeweller's screwdriver shaft.
It is best to trim it after you have wound it smaller because there is inevitable damage to the coil-ends from the pliers.