Wouldn't it be easier to use fiberglass, make a wooden template of the finished article, smooth off using car body filler, then when happy with the plug, fibreglass it up, a lot easier than trying to get plastics welded up at joins etc...or buy a ready made off the shelf pod, like nasa clipper pod or the lewmar type, the latter expensive..

Plenty of informative videos of fibreglassing on youtube, if you have no experience, it is quite simple to perform a decent fabrication...