I would add that thick cushions made of soft squashy material must be avoided because of the risk of sinking backwards/downwards under high G loads, which can then cause a pilot to involuntarily pull back on the controls with extremely nasty effects.
Many gliding clubs use rigid insulation foam "spacers" for shorter pilots. These are fitted between the rigid seat structure and the normal (thin and preferably dynafoam) cushion.
I'd advise that you bear the point in mind during your search but I can't help more because I'm not familiar with your aircraft.