As far as gun bags go they make padded speargun bags for any length gun. Just search for any website that specializes in spearfishing/freediving. (Hanapaa hawaii, pirate scuba, blue ater hunter, nor cal freedive etc). I think they start around 75- or so, cheaper than rifle cases/
These sites will also have info for building your own guns. Most people who make there own build teak guns. You could make an alluminum or stainless tube gun as well, but a teak mid-handle might be just what the Dr. ordered for rig diving down south.
It's easy to pick up trigger mechs for a wood gun and the main ones all rate really well for loads before failure. There's a ton of info about this sort of thing on deeperblue.net in the forums. Good luck.