Thanks for the compliment Sc...it's just that I can't get out of the habit of viewing mouldings with aviation standard eyeballs!
I shall try to get as much of the equipment up front to minimise any balance issues but, as you presume, I do plan to fill the rear 'tails' with foam after I've made up the rear power pods which will be mounted one under each side. These will be a small brushless motor in a sealed case, probably turned up from delrin rod and with a lip seal for the motor shaft and a direct mounted prop.
The full size craft has a pair of cylinders running the full length of the machine inboard of the bladder casings, these are for the breathing air and so on this model that will provide some buoyancy, especially to keep the tail up. The cabin hemispheres arrived today, these are 150mm outside diameter and as can be seen they are very nicely blown from 3mm sheet, the 20mm flange will be reduced after attaching the mounting/sealing rings which will be turned up from some flat sheet. Not sure yet on what to use though it probably will be Whale tufnol 12mm thick, being easy to machine, tap threads, strong and not too heavy. The intention is to make the cabin functional in that the Rx and small camera and some other gubbins will be fitted there, but instead of the upper shell being hinged as on the full size, it will be bolted down onto an O-ring located in the lower half-shell.
So here are the bubbles, the next job will be to brush a good few coats of PVA mould release onto them for protection during the build, it'd be a shame to get them scratched!
Trevor