cheapest housings are on ebay, like 50-60 bucks though is nothing to sneer at.
the issue is that the camera has a convex lens too, so the existing lens should be kept in place as it accommodates the bulging of the lens...what is more, the curved housing does expand the field of view on land so it serves that purpose as well. that last bit is something of a double edge though, I wouldn't mind a little less fisheye and depth of field in exchange for a larger image. ideally you could keep the existing one and add a flat lens for when you wanted it.
thanks for even thinking about this, hope it turns out to be a quick fix because you could make a fortune with it <img src= " height="23" srcset="https://spearfishing.world/forum/images/smilies/emojione/1f606@2x.png 2x">