Progress update…
After going in circles with a 3D printed design (FD#2/underboard) - was going to use angle aluminium supports around the base plate bolts. But concerned 3d printed plastic will warp. I will loop back to this as I like the concept and it gives flexibility in design… just think I need to get something working first which is more durable and watertight, so…
Now switching to rectangle aluminium tube: 100x25x1.6. Initially will try doing this with brazed supports (10mm aluminium tube) for mast base plate bolts.
This should be similar to FD #2 Slim size. But 25mm narrower and with twice the battery capacity for only another 20mm or so in length. That’s the goal anyway 
Will have 2 x 6s2p battery packs - fore and aft giving 12s2p.
Biggest challenge will be closing the ends - making them water tight. Current plan is to make a silicone seal on flat plate ends and braze on some connector supports to the main body Any other suggestions?