For easy manufacture of 2d structures, I recommend water jet cutting. If you use ABS plastic, you can solvent-weld parts together with acetone to make airtight seals and more complex structures. Since I use mostly metal parts, I just make threaded screw holes for the pieces and screw them together. ABS can also be thermoformed (heated in oven and pulled around a mould by vacuum), which would be good for air chambers, covers and all sorts of thin, hollow shapes. Of course these can be also solvent welded to water-cut or 3d-printed parts in ABS.