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DIY? An ebony or other ultra-hard cap is unnecessary, given the lower tension and larger diameters. So, it can be (and should, according to many) be made out of a single piece. People have different taste as to sound, but getting anything close to typical banjo sound from nylon requires a lighter and less stiff bridge than with steel strings. Less stiff allows a softer wood than maple, which would be easier to find and easier to cut.
You can use any bridge for nylon strings. If the slots are too small you can use a narrow file, a folded piece of sandpaper or a utility knife. There have been endless discussions about bridges for nylon strings. Search the archives and you will stay busy for a week. While any bridge can work, you will find that some sound different than others. Here is one from Elderly that many like for nylon.
elderly.com/products/morris-no...194508352
I like all rosewood or ebony.
The Morris is good, I have it on one of my banjos. An ebony-capped bridge is not ideal, but the banjo police are not going to come after you for using it with nylon strings. If you feel like experimenting, you could take a standard bridge and sand down the faces to thin the profile. You could also make the feet narrower (or even cut off the middle foot entirely).
You can use any bridge with nylon strings, as long as the slots are wide enough.
BUT, they'll sound dull and muted with a bridge that's stout enough to support steel strings.
OTOH, what I call "dull and muted" (and to my ear is lacking in exactly the characteristics which make it sound like a banjo) may be what you call "mellow and plunky", and be exactly the tone you want.