From an industrial perspective...and there is absolutely no reason not to adopt the same orientation or engineering principal for model work, is when making a split piston seal to adopt the
Z cut approach
This when installed on and over to the groove a one piece piston, is a sealing element that will be pressure energised to seal in both extension and retraction
The example here appears to be a woven Teflon square section braid......
Derek
[PS...some 50 years ago as an apprentice I remember using this
Z cut approach for 50" ID leather V packing's over a large diameter wooden former, then let them soak in Neetsfoot oil for a month prior to the staggered installation of a new set of gland seal packing's to a 95/5 fluid vertical extruder press ....the continuous coil of preformed leather packing was of a diameter being taller than me
The real trick was to calculate {a
2 + b
2 = c
2} and mark out the distance between the two vertical lines that when cut to form the
Z , would provide the packing with the actual 50" ID]