I have a 138 with the old version of the original 5x7" carrier, the one with AN glass in it. I never liked it much. Yes, the focus/calibration aid in the margin is nice, but I don't find it necessarily essential, although it does help aligning the baseboard to the center of the optical axis. However, that can be done in other ways as well.
I just don't like glass carriers I suppose, and the drawback with the negative carrier in this enlarger (and many like it), is that if you want to use several film formats, you'll have to search a lot in the second hand market and have to have a good dose of luck in finding the parts you're looking for. More often than not, they're IMO ridiculously expensive and often in a poor state or incomplete. This is why I resorted to DIYing a set of holders/masks myself. Instead of carboard (which works, but is a little flimsy) I just used some MDF that I had lying around for other purposes. As long as it doesn't get wet, it's pretty dimensionally stable and easy to work with (the dust sucks though when you work it).
What I did was make a base holder that slides into the enlarger with some guides glued to it so that it always sits in exactly the same place. The fit is actually better this way than I get with the original holder, which has a a few mm of play. This base holder has an aperture cut into it, centered around the optical center, that is a bit larger than the largest negative format I want to use (4x5" in my case). I then made inserts for the different negative formats I use, e.g. one for 4x5", one for medium format (still working on 6x4.5, 6x6 and 6x12 variants) and one for 35mm. These inserts are again made out of MDF, with a mask that is appropriate for the format. I made the masks out of matte board or used standard Durst masks if I had them for the format. The negatives are held down with strips of steel that click onto magnets that I glued into the MDF holder, so that the negative is effectively clamped down and held in place by the margins. It's a very basic variant of the tension system that the more fancy Durst holders use for e.g. 35mm, but my contraption doesn't tension the film (I find it's not necessary, but technically it's a better solution as long as it doesn't damage the negatives, like a Beseler 4x5" holder I was gifted and that horribly scratches the negative margins...) Hence, all my holders/inserts are glassless, so less problems with dust and getting AN glass. They're also much easier and quicker to use than the original holder I had.