Going from 11x14 to 16x24 or 20x24

Sometimes barbarism is the only answer....

Here’s a viable alternative to barbarism.

Rather than ruin a perfectly good negative carrier, simply make a temporary one of mat board or thinner card stock. I do this all the time with old roll films of sizes not made in 40 years or longer.

If you need to keep a negative critically flat in a glass carrier, that too is easy. I’d choose a larger carrier, say 6 x 9 cm or 4” x 5” and cut an oversized-window mask from a piece of scrap FB print. The window is easily cut with a single-edged safety-razor blade. Lay the window mask centered over the negative and close the carrier. The glass carrier will keep the negative flat and you’ll project the jagged black edges you want without permanently ruining a good carrier.

You can also experiment with differnt size borders by making masks of different opening sizes.

What Ian said. I have a large collection of mat board negative carriers of various odd formats that I have made over the years. I also wouldn't hesitate to enlarge a 6x7 negative to 20x24 or bigger, but of course it depends on the image in question. There's no general rule against it in my darkroom anyway.

20X24 would be near or maybe even beyond the height capability of the 23C. Crank it all the way up, focus, and see if the image is big enough to fill the paper. An easel for 20X24 would probably have to be home built, but that needn't be too complicated. Exposure time increases in proportion to area plus an adjustment for reciprocity failure, so you may be facing very long exposures. Sink space is usually the greatest problem with big enlargements. 16X20 can be handled in trays in many darkrooms and in rotary processing tubes just about anywhere. 20X24 gets difficult, but you can probably build a suitable tube and some rig to rotate it on (hand power is possible). With bigger paper, handling time increases, and that causes some additional problems. Particularly with tubes, you may need to dilute the developer to get longer processing times, Or you could select a slow developer. Having the extra time will reduce splotches and irregularities that may occur while changing solutions. You will have to test to determine development times and how dilution affects contrast. Big prints require more solution, too.