After a few searches on eBay, I found exactly what I wanted. A pair of wheels for a grass cutter, designed for a half-inch axle, and for just twenty-something dollars. I found others on eBay that almost fit the bill, but they had 'nylon bearings', which are not really bearings at all. If you don't find what you're looking for right away, on eBay or Craigslist etc. then don't give up - you'll find them. It took me a few days to find mine. You might even try lawnmower parts specialists.
|A pair of grass cutter wheels found on eBay|
You might want to hold off fixing the wheels to the axle until after the two parts of the frames are joined, depending on how easy it is to unmount and mount them. I couldn't wait - I wanted to try out the wheels. To secure them, I had planned to use large washers and a couple of split-pins through holes drilled in the tube, but in the end they were a good tight fit and didn't need anything apart from a couple of shiny press-on caps to hide the tube ends. With the tube secured in the vice, I used a heavy rubber mallet to avoid damaging the wheels.
|Tap the wheels on to the axles with a soft-faced mallet|
Ensure the wheels turn freely, and double check alignment and camber (rolling angle).