So how will you be joining them?

CA glue ups? At registrations pegs to align it. Make the pegs slightly tapered so they start easy. Prusa slicer let’s you cut and add them if you need to print a thing in two goes.

Screw of some type? (I like socket head m-whatever bolts they look nice. If you counter sink a space for the head.)

then you got options. I’d recommend getting a tap to cut threads for low strength joins. You can cut thread straight into most plastics and this will also adjust a slightly underside hole.

Alternatively for much stringer joins, use heat-inserted thread (they’re brass beads you press in using a soldering iron.

Either way it’s generally a good idea to have over sized holes on one part do they slid freely into the threads and tighten down.

Also, tapered pegs can be used without glue- if you make the leg’s base slightly over sized and the tip under sized, friction will hold it loosely.

If you need a seal on the bolt down parts, you can make o-rings using air tubing (for fish tanks air lines,) and silicone caulk coming out a caulking gun- if you use an air compressor blower nozzle after it cures, ot comes right out. Or you can make them hollow by blowing compressed air through before curing.