Try [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXJBU2&P=0]this[/link] one. (Prebuilt)


Or [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXJBU0&P=0]this[/link] one. (Requires screwing on handle and battery cover)
I had a field box with two casters on the front as you have and a set of wheels from an old skateboard on the back. I towed it around on a grass field for a few years until I needed a bigger one. It worked great.
Sadly they don't take AMEX but they do take Paypal. If you don't have a Paypal account go to PayPal .com and sign up. Its free and I think you can set it to take the money you spend out of your AMEX card. PM me and I can help set it up if you want to.
Chuck