Yep, small reclining car seat, if you can fit it inside. Not many do. We used one like that until our son was about 9 months old. We dismantled the built in seat in our Burley Solo and simply lashed each corner of the car seat securely to each corner of the trailer. For gentle riding on paved roads and smooth gravel roads it was great, he lay there reclining like a prince :-D

Once he outgrew that seat the next size was too bulky to fit, and didn't recline much either, but he was pretty much big enough by then to use the regular seat. The car seat solution was lot better and more comfortable for him though.