sadpanda.us/images/686767-NM8SOWJ. j p g (photo of baby trailer with modification for car seat) I rode with my baby since 6 months I bought a used Instep trailer for $40, took out the fabric seat back for later, and mounted a plastic tray in the bottom made from a sterilite bin. I'd recommend plywood though! This worked with lots of zip ties, and the weight is supported on the heavy bottom just like the kid's butts would be with the 2 seat fabric back/seat. She loves it! She has been excited about it since day 1. She sleeps and plays in it, and gets out at lots of rest stops (picnic benches, scenic areas, friends houses etc).
When she was 6 months, we just went along like a stroller pace, 2-5mph.
We go about 5-10mph on bike paths and crawl on some smooth nature trails. Pulling the trailer is a lot of work, so usually about 8mph. I put 34tooth granny gear on the bike. We ride about 5-20 miles, usually about 3 hrs total with stops and 10 miles.
I tested it out by putting a 40 lb bag of salt in it before I used it. I couldn't tip it over if I tried, but I guess if you hit one wheel on something or dropped off a curb etc you might.
I put SLIME tubes in with BMX tires for a more cushy ride, and have ordered some Amerityre polyurethane tires ($12 each in 20x1.95, yellow only at sale price, noflattires net, call them)
Now she is getting big and walking, I am going to put the fabric seat-back in the trailer.