My old sled is made from a Jet Sled Jr. and a plywood box on top to make it the right size to sit on. It was sized to fit in the back of a Pontiac Vibe sideways.
I used framing splices from Home Depot to secure the plywood on top of the sled. I customized it over the years to carry rods, pole holders, my shovel, drill/auger setup, spud for checking thin ice, and of course the Ice Shanty flag. I still use it for early ice, but mainly use a snowmobile with a custom box when the ice is thick enough. I did make a Smitty, but it was more for when the ice is rough, and I am pulling behind a snowmobile. I have punched holes in the sled on rough ice before. The Jet Sled is really too thin for towing at speed.