I also made something very similar by buying some 25mm square ali tube and some 16guage ali sheet.
I used a Workmate to clamp the sheet and a length of wood to fold over the edges to make a sort of tray that sits in the tubular frame, with flanges going over the top of the frame.
The whole lot was pop rivetted together and some heavy duty castors bolted on ( I got mine from RS ).
It doesn't weigh too much, is unaffected by rain and has lasted for about 15 years so far and is still in excellent nick.
It's worth getting a pair of non-steerable wheels ( they're much cheaper anyway ) and a pair of steerable lockable castors. You'll be able to steer it very easily and the brakes will ensure that you can leave it on a gentle slope with a fair chance of it staying put.