The problem has been solved several years ago. First you DO need cradles, second you need the handle.
Now, the handle comes with bearings, just like the wheels, but you don't want the axle to rotate, thus making the cradles turn upside down.
So, you put a through bolt in to keep the cradles right side up.
Then it is so easy. Just lift the bows slightly to get the cradles under then slide them all the way under.
No need to tie anything. Sure, there is the investment, but think of all the money you save from going to a doctor with a bad back.
Here is the link to the handle:
http://www.onlinemarinestore.com/store/pc/viewPrd.asp?idproduct=14184&idcategory=287