Search this website for "cart" or "dolly".  I did a variation of the DIY kayak cart, scaled up, for my skerry, and while it wasn't perfect, that was mostly my execution!  I did it with mostly scrap 2x4s and a couple of spare lawn mower wheels.  If you found some larger garden cart wheels, that might be easier on irregular ground.  You can find balloon wheels if you are on a beach.

The page is https://www.clcboats.com/shoptips/fitting_out/kayak_cart.html

   Rather than foam, I wound up using indoor/outdoor carpet scraps to cushion the bunks.

For our PMD, I bought the tongue-less boat dolly CLC sold at the time (they have different ones now).  It was a similar concept to the Yak Kart shown in those plans, except for having fat, low pressure, pneumatic tires.

The trouble with the PMD is that she has so much rocker to her bottom that she wanted to keep sliding forward off the dolly before I could get her strapped down, squirting the dolly out behind.  That was somewhat resolved by removing the wheels, which both lowered the supports and kept the dolly from rolling.

The good thing about it was that it could be knocked down and carried in the boat if needed.  It was stout enough to carry the boat, even loaded with sail rig and gear, though the plastic risers for the hull supports had a tendency to twist (again, a very curvy hull form, she is) and unlatch from the sockets, a problem solved by a bolt through each  joint with wingnuts.  I think the ones they carry now might be better that way.

Once she was strapped down to the wheels, it was a piece of cake to move her around, even some distance over sand.  I often dream of taking the boat down to Cape Lookout National Seashore, using her to transport my camping gear down to the Cape, and then using the boat_with_dolly assembly as a cart to shift my campsite from time to time.

.....Michael

   This is my Skerry on a homemade hand cart. The cart is a frame of angle iron with a skin of exterior plywood covered with outdoor carpeting.  I've been using this for 15 yrs and it still works great!

- Bob

   dang it, let's try that again

   The passagemaker is just 90 pounds. This cart is what we now use for kayaks and canoes.  It has a capacity of 200 pounds. It is wide enough to handle my Jimmy Smith II.  The nice thing is the tires don't need air pressure.  Straps are attached to the cart with velcro straps to bundle them up for storage.  Wheels come off for loading on the boats.  

https://maloneautoracks.com/Clipper-TRX-Deluxe-KayakCanoe-Cart-with-no-flat-tires.html

 ".......  dollys and carts I've seen are either for canoes and kayaks or cost over $500......"

You are looking in the wrong places.