Naish foil mast waterlogging fixes?

[Stoke]

I'm riding Naish Thrust foils and have a repeated problem where the hollow mast fills up with water during my session and gets extremely heavy.  It makes a very noticeable difference, you can really feel it when you pump or carve hard.  After the session the water stays trapped in the mast and drains out very slowly, covering everything with salt over a few days even after I rinse it.  Yes, I could disassemble and rinse everything after every session but... does anyone actually do that? Plus, that doesn't help the mid-session waterlogging.  This happens with both my 70 and my 90cm masts, and I've heard the same thing from other people so it's not just a single faulty part. 

I have been thinking about blowing the hollow mast full of waterproof foam to keep most of the water out in the first place.  Something like this:
https://www.amazon.com/GREAT-Insulating-Sealant-Great-Stuff/dp/B01LZGCNTH/ref=sr_1_fkmr0_2?ie=UTF8&qid=1534305124&sr=8-2-fkmr0&keywords=Dow+Great+Stuff+Pond+%26+Stone+Insulating+Foam+Sealant

The downside of trying this being that it will be a pain to dig out if it doesn't work or absorbs water and gets permanently heavy.  Alternatively, I could do my best to cut out a chunk of hard foam and insert it, which would help but it's a long weird shape so I probably won't be able to fill it completely. 

Any other ideas?  Other people have this problem?

[Bean]

The regular Great Stuff actually absorbs water like a sponge, so whatever foam you use, test it first.

[PonoBill]

Two part foam has sealed chambers, but it will be messy. You don't need to displace all the water, just most of it. I'd shape some blue construction foam and press it in. Maybe make it in short chunks and press it in so it self-forms.

[sharksupper]

I made some custom gaskets out of very thin rubber, have yet to test though.  They were easy to make and seem like they'll work, we'll see.