Make the seat VERY firm. Otherwise you don’t actually stay properly elevated. Heavy, thick felt is a good fill for doing a cushion type wedge. Square shape is fine. Just don’t want much give under you in case it affects your position. Like those too soft couches that are hard to get out of!
We made a square wood frame with board seat (or thinner plywood), slight padding and cover to try various wedge heights. We made our first couple wedges like that. When it was the “right height” we stapled naugahide on the sides and some black suede with polyfil for cushion under, on the sitting portion. No curved edges or sides because there was NO sliding on it! Really was grippy! Ha ha
We copied an antique wedge seat that came on a vehicle, too moth eaten, mouse nested, to actually use anymore. Have since gotten a couple more antiques just like it on other old vehicles. So we figure it was a common wedge style “back then.”
Something else to consider with being higher: is do you need a foot rest or foot rail to have solid footing? Have to brace yourself on something while seated!
I had some old but nice brown reins we used after adding simple buckles, punched holes to fit, for running around wedge and board under seat. Looked nice, wedge sat quiet in use, FREE from stuff we had already.