Following 
Btw, my experiance with those waterproof XT90 is not great, the seal is quite small and easily damaged. Make sure to grease them well.
How will you hold box to deck?
What is your planned capacity?
That’s disappointing. I should have expected it…
Nevertheless, these connectors are an overall improvement.
It can be further enhanced if time allows: the seal slot depth is 0.75 and the wall thickness to pins is 1.35, with a difference of 0.6mm, so it’s better not to make the slot deeper. However, the slot can be made wider with making a wider silicone.
Grease, yes.
I’ll also have another overall seal at the battery-to-controller entry.
Capacity
12s2p and 12s3p, with the latter being longer (board lengthwise) by one cell. Will start with 12s2p.
Attaching to the deck
I’ve already considered dozens of variants…
By the way, it will also work when attached to the user’s back.
The answer also depends on what to do with the first prototype.
Even the first prototype should provide:
a. The ability to slide in a new battery quickly
b. The ability to move the rig forward/backward with minimal effort to find the sweet spot
c. Preferably not tearing the deck cover
With that said
The current test plan for board attachment is:
a. Place a 1mm aluminum or carbon fiber 20x40 sheet underneath the rig
b. Attach glued points under the front and back (board lengthwise) edges of the rig. The controller is screwed. The battery attachment point can initially be a simple nut glued, but ideally, it will be like a bolt in a glued nut so that the battery slides on top, pressing it down.
c. The sheet is stretched horizontally - for example, with a ratchet, but smarter. If needed, the sheet will have some attached rails to prevent bending.
Sheet thickness can be adjusted later, and a variety of other ideas will be tested if necessary.
Update regarding possible improvement of this XT90 seal:
Female part has a feature that the lower body is more narrow, like seen in this picture. Male part slides 1.2mm on top of this narrow part till the battery body. So in this place can be added (an even wider than male side) silicone seal such that it will squashed when connector attached.
