You see some really over engineered sledges. I think the best designs for the uk have plenty of contact with the snow. It is very rare that you get deep compact snow that makes the runners worthwhile. The ones with “brakes” always seem to be broken and besides, brakes are for wimps.
If you were going to make one then something reasonably flat with a simple non mechanical steering mechanism would be good. Those traditional wooden runner types do work on the piste. They are kind of like skiing in a permanent snow plough so by shifting weight you can steer. Dunno if you could do this but with a bigger surface area so you don’t sink.
We used to go snow boating in kayaks. That can work and you can steer by edging on those with a proper flat bottom and sharp edges (like skis/boards). You can also use a paddle 😉
So in summary plenty of surface area to float on the snow. Some adaptation of runners (not parallel?) so you can steer.