A very cheap and very functional option would be just to add a flotation sleeve on a pipe. This sleeve could have hooks and connections to reel or other roller systems. Would also allow for heavier shafts and still keep guns nice and weightless. Pathos has one for their guns ... but really it is only an after thought and not really well designed. I remember also discussing that with Dano from Mako spearguns and just looking at the Mako website I see they have them here
https://makospearguns.com/barrel-float/
That is a very good low cost and very functional option ... but of course a custom cuttleshaped CF one piece tube looks much nicer ... but at a much higher price.