I did so.
Quite easy - I found a 35mm neg carrier. Measured out the dimensions of an Xpan negative, and drew it onto the carrier.
A friend at a local high school shop class had a plasma torch, which he used to cut the hole very precisely. I then used a file and metal sandpaper to smooth it out.
It works like a charm, and gives a unique full-frame border that I can choose to include or not.