Weight balanced on top of the wheels (gametote) like that will make it harder. You have all the weight going down on the wheel and a high center of gravity making it want to fall over.

a wheel hanging behind splits the load part on the wheel and part on you but also lowers the CG. If you get a large wheel then use a pair of bent arms to connect to the axel then come downward you can get the weight below the axle and make it more self balancing rather than wanting to fall over.

A quick image search found [url=thishttp://www.eaglematetrailers.com/images/empty_trailer.jpg][url=thishttp://www.eaglematetrailers.com/images/empty_trailer.jpg][url=thishttp://www.eaglematetrailers.com/images/empty_trailer.jpg]thishttp://www.eaglematetrailers.com/images/empty_trailer.jpg[/url][/url][/url] not a bike trailer or backpack carrier but see how the frame drops down below the axle? Use a larger daimeter wheel and drop the frame down some and put the weight down low making it easier to balance. Then the length of the bars coming up to you act as a lever laving the weight less but also the whole amount isn't on the wheel either