I bought a Burley bike trailer off of Craigslist and love it. It came with a handlebar, but no stroller kit. When I went to buy a stroller kit, I was pretty disappointed by what Burley sold. Their wheels are plastic! Based on the overwhelmingly negative reviews on Amazon, I decided to see if I could replicate the same basic design for about the same amount of money, but with much higher quality materials.   Here's what I came up with.
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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

The bar that connects the trailer to your bike is 1 1/8" wide. I looked all over for 1 1/8" aluminum tubing, but I could only find 1" or 1 1/4." I noticed though, that if you remove the plastic inserts in the housings (seen in the lower one), a piece of 1 1/4" tubing would fit nicely...except for the roundover. The roundover is a 3/16." I didn't have a 3/16" roundover bit, so instead I took an old table saw blade I didn't care about anymore and chamfered an 1/8" off the edges and rounded them over with a file.

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

At this point you can cut your tubing on the chop saw. I found that 13" looked about right. You can also put the furniture end stops on their fronts at this point. My hardware store didn't have the exact ones I needed. I found two, though, that would work if I pared off the little ridges that go on the inside of the tube. They even had 3/16" round overs, just like I planned....

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

You will only have 1/4" of the caster's stem projecting above the tubing, but it's enough with a flange nut. You'll need a pipe wrench or something to hold onto the caster as you crank down on the nut.

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.

Step 4: Take a Stroll!

Congrats! Your trailer is now a stroller. And instead of buying Burley's flimsy plastic wheels that might last a year, you have pneumatic wheels with a 300 lb combined load capacity (which match the look of your other wheels) mounted on structural grade aluminum. And you saved eight bucks to boot. The only disadvantage I can see is that you have to remove those plastic housings and keep track of them. And without them, inserting and removing them isn't as a smooth. Small price to pay I say to feel like a badass when I pass a pack of stroller moms with my latte.