Materials, Cost and Time:
1 Aluminum 6063 square tubing 1 1/4" x 1 1/4" x 36" ($15.73 from Amazon)
2 6" Threaded Stem Casters, black pneumatic wheel, 1/2 - 13 x 1 1/2/ ( $62.83 incl shipping from Access Casters)
2 Black plastic furniture end caps ($2.00 at the hardware store)
2 1/2 - 13 Flange nuts ($.20)
Total Cost of the hack: $80.76
Cost of Burley's stroller kit: $89.00
Time: One long baby nap (~2 hours)
Step 1: Rounding Over the Aluminum Square Tubing
When you start fitting them, it helps to adjust the housings' by loosening their screws until they align better. Once they fit nicely, clean up your roundovers with some medium sandpaper
Step 2: Drilling Holes
Layout your 1/2" holes on opposing sides of the tubing for the wheels. I centered my holes about 2 1/8" from the front. If you aren't using a drill press make sure you punch a little divot where your centers are so your bits don't wander. And use some sort of light oil, bike chain lube, or whatever you can find as a cutting lubricant. If you want to make your wife mad later, when she asks you where the olive oil is, you can tell her it's out in the garage.
Step 3: Attaching Wheels
Once they're attached, fit them into the trailer's housings and mark the holes for the pins. I found mine to be about 1 5/8" from the back. Choose a drill bit that is slightly larger than the pin because, contrary to popular opinion, you're not perfect.