I don't use anything to hold the two or three sections together, simple pressure fit, and not much pressure required. The bottle is roughly cylindrical prior to cutting, the external curve of the shoulder pressure fits readily inside the ID of the main portion of the trap.
The trapped bubble is not large, but enough to run the length of the straight section of the base/trap. This keeps the assembly horizontal just at/below the water surface. Small critters in the plants near the surface can readily make their way into concave reversed bottle top section, through the neck and into the trap.
And you leave the caps off, that is the "gate" or entryway into the trap, concave to fish/shrimp coming in, convex to those already trapped - which is the basis of the technique. This is all modeled after commercial bait traps which use conical sections of wire mesh.
HTH