I cut out my glassless carrier to get the full image area, minus the clear part of the neg obstructed by the rails in the film holder. I think I used a Dremel, then finished with a file and sandpaper. I would tape the neg to the carrier putting a bit of tension on it to keep it flat. The worst was that I then found out the opening above the negative carrier on my Omega D5 was causing a little vignetting. That took a couple more runs with the Dremel. And now I don't even use that enlarger anymore except for proof sheets.

I did make a 5x7 conversion a while back on a Durst L1000 and made a frame that held two plain pieces of glass. I masked those down to exactly the size of the full 5x7 negative and so could print the entire thing. The tape had the added bonus of adding just enough separation to prevent newton rings. That would be equally practical for 4x5 of course.