I think I'm one of the more recent people to try and solve this. For only 25lbs, definitely think about a seatpost mounted one. They go up to 25kg in some cases (Blackburn, Rixon&Kaul, Topeak...)
For a more traditional approach, mine isn't ideal, but it works. Ideally I'd like longer upper stays so they could be P-clipped to the top of the lower triangle.
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Another option is to replace the suspension with solid rod, and then you can clamp the upper stays to the seatpost.
Finally; I never figured out a rack for underseat use, but I did bodge a 2-bag method if they balance each other. The straps drape over the horizontal bar supporting the seat. Once hooked on they were pretty secure but they do clatter about a bit.
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