Try -

http://www.kask.org.nz/do-it-yourself

and / or go to -

http://www.kask.org.nz/kayaktrolley

Golf trolley wheels are good enough for a loaded double so if not strong enough for you, what are you carrying? The Queen Mary?

Keep the height of the trolley to a minimum for stability. Also strap it on to the kayak so if the kayak is lifted (resting on one end and lifting the other to manoeuvre) the trolley stays with the kayak.

Freya sells the same thing in Germany but despite me telling her, still doesn't put the prop-up legs on them. This is the problem when someone takes your good design and doesn't do it properly...

Alex