Custom setups?

I wanna try messing with some chess variants that're just the normal rules, but with piece substitutions. For example (not that I want to play it, just as an example) replacing rooks with a second pair of knights, or some fairy chess piece. Is this possible? All I can find that changes the pieces are 960 (which doesn't change the number of pieces, just the arrangement) and variants that have fixed setups and aren't always on an 8x8 grid. Is there some option I'm missing that just lets you arrange your own board of whatever size with whatever pieces (fairy or not) you feel like? I don't want to mess with the rules, just the starting position.

And yes, I did google this beforehand - I'm not finding anything other than something about a practice mode that's also under Daily, but I think that's outdated. I can't figure out what it's saying to do, at least, and it really feels like what I'm looking for would be under "variants."