I wouldn't worry about sitting so much - other than Han, you didn't see any seated stormtroopers in any of the movies. Whenever I'm in armor, I always assume I'll be on my feet the entire time.
If you must sit, however, perhaps you could use strapping that is of greater elasticity. Another alternative is either undo the butt-to-crotch connection and flip up the buttplate, or take it off altogether.
Snap system instructions:
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