Beseler does make one for the 4x5 enlargers -cost is a whopping $233.50 from B&H
They do not make one for the Beseler 23C which I use. So I sent SK Grimes a regular 23C 35mm carrier & a sample xpan negative and had them machine it out to 24x65mm. Cost was $65 back in 2006/2007.
xpan negative carrier.jpg