That is one ginormous seat bag which makes a seat post mount difficult.
Mine is half to a third of the size and contains: Spare tube, tire levers, tire boot, 3 patches and glue, multitool, valve core remover, 2 CO2s and head. I can affect any repair with what I carry. Yes it’s all crammed in there, but all of it fits, plus it is a bit more aero.
Consider a smaller bag, pack efficiently and then a seat post mount is no issue.
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Road 🚴🏾♂️ & Mountain 🚵🏾♂️