When you say “too big”, I assume you mean the backs of the foot wells are too far away relative to the seat. Many complain of too big meaning seat is too big and they pad the seat to fix it. If footwells are too far, you need to effectively bring them closer to seat by building up the backs of the footwells (where your heels press). I do not see how building seat up makes it shorter. Look for something you can cut to form, but not spongy. Speaking of yoga, yoga blocks are great as they give in very little to pressure, do not absorb water (closed cell?), and you can cut them fairly easily. Cut a piece the width of your footwell (I assume two, narrow footwells), so about 4”/10cm width, and about the same length/height, then slice it to whatever depth you need. I say you could go up to about 2”/5cm thick before reaching pedals is a problem. Round the bottom corners to fit, glue it on (hot glue so you can remove in case you need to). You will have to angle your peddles back towards you more. Good luck.
KR McGregor Rhythm, V10Sport, Swordfish S, Fenn Tarpon S, Fenn XT, Twogood Chalupski, Findeisen Stinger spec. Had: V12, Stellar SE, Huki S1-X, Burton wedge2, Fenn Tarpon