There are various ways to tackle your dilemma, one is to mask the glass with tape, bit of a pain. Another way is to make an insert into another neg carrier that has a bigger negative size than your Xpan negatives.
I did a work around with my DeVere enlarger for my Horizon 202 negatives, 24x56 (nominal) size. I had a friend with access to a Laser cutter who cut a blank from a mild steel sheet to fit exactly into the 4x5 neg carrier, then another cut was made to the exact frame size of my Horizon 202 minus 0.25mm all round.
By making the negative carrier hole ever so slightly smaller, I am able to make direct prints without any black edges showing on prints, unless I'm doing border less prints.
This has been very successful.
The only downside is that the negative carrier is quite heavy when the steel inserts are placed. You will need an under and over set of inserts, by the way.
Laser cutting of mild steel is reasonably cheap these days, certainly cheaper than when I had mine cut in 1994/5.
Mick.