Minimal ingress I would imagine. Once airborne next to none. Humidity would be more of a real world problem? Do you have fuel drains?
I'm going to talk with John again shortly, I've asked him about the belly addition, but I was thinking more for a bit of luggage, as there is only enough room in the cabin for a penicil. and pilot.
I thought about doing something like the 90 ltr supapup tanks, but yes weight is an issue to be considered. I'm still new to the kit, and kit building scene, though I was an engines-airframe lame years ago, before I moved onto other things.
What happens if you want to build an A/C over the 340 kg? I'm imagining -340 is LSA? like I say, I've got a lot of work to do before I understand what the hell is going on in sport aviation.
What engine is in yours? Mine came with a BMW R1200S oil-air cooled engine, and a Hirth reduction box. It's a horizontally opposed, dual spark plug, air and oil cooled, fuel injected twin that make 120 hp, so it ticks a lot of boxes. and weighs about the same as the 4 cylinder Jab.
Cheers.