I've recently been instructing a diminutive girl in a Puchacz. The pedal reach problem was solved by carving a seat backrest from a block of 150 mm thick rigid foam insulation.
Large thicknesses of soft cushions are a big no no because of the risk of excessive compression under G loads and your losing contact with the controls.
She also has a minimum cockpit weight problem which was not met by the normal glider ballast weights and had to be cured by a lead shot ballasted seat cushion secured to the lap strap attachments.
The key issue is that that the ballast cushion must not be able to slide forward in the seat and interfere with the control column.
I have also seen heavy seat cushions made from heavy but flexible X ray shielding material or sheet lead can be moulded to the shape of the seat.
I don't think there is any current 2 seat glider which you would fit without taking similar kinds of measures. You would have fitted the old T21 OK!
Whichever club you go to, talk to the instructors who may in turn have to speak with the club glider inspectors. They should be able to come up with a solution.