Honestly I wouldn't even mess with your flotation chamber. I would instead consider putting in an adjustable bow seat. That way your front paddler always has the leg room He/she wants and to top it off you can use the seat to adjust boat trim for varying paddler weights and varying wind conditions.

I know Wenonah has them available.
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There's also some different styles out there.
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This is the only DIY kit I found after a couple of searches.
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Otherwise I know there is pretty detailed instructions on how to build them from scratch in theses two books.
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Good luck
SW