Find yourself two wheels for a hand truck or wheelbarrow. You'll want to get the widest wheel possible. Unfortunately most are about 4", 8" wide would be superb. If you've got a H18 forget about using something 4" wide, you'll never move it, especially on sand.
Knock the bearings out of the wheels and use a round file to "straighten", or remove, the taper. Then it's off to the hardware store (with one wheel) to find the largest piece of piece of metal pipe that will fit inside for the axle (a heavy wall that won't bend easily) and a piece of plastic pipe large enough to slide over your metal axle.
Next is the creative part. Something is going to attach to a 8 to 12 inch piece of your plastic pipe to make the cradles.
Fiberglass over wood with carpet pad?
Aluminum with carpet?
Parts of your kid's broken plastic toys?
This is where we get the input from all those superior minds out there (Sailors are such resourceful people)
Two short 2" pieces of plastic pipe are fixed (screwed, glued) to the metal pipe to the insides of the wheels at your determined width, slide on the wheels, slide on the cradles, bolt through the metal pipe? to keep everything in place?
There are a lot of variations and possibilities here. I'm interested in hearing ideas.
I haven't located the "right" wheels just yet. Harbour freight had their (what looked to be 4" in the ad) dolly wheels on sale for $4.99 each, might be OK for a 14.