I made one for my BCS850, dolly attaches to the pin coupler above the PTO. Mine is very simple, a straight bar(channel like road sign post)about 4'long, axle about 25" with foam filled wheel barrow tires and a piece of 2 1/2" square tubing vertical with the ball on top. The ball is mounted on 2" tubing, removable so I can change it out with a seat. It works, works better pulling than pushing, in the yard I use it for moving a log splitter and light trailer. If I were to make it again, I would use the PTO coupler with a short "tongue" and a castor type wheels, power barrow style but shorter. Should work good for tight flat places. I just don't have much flat.
View attachment 562740THis picture shows the dolly with the seat in place pulling a wagon. Seat removes, insert ball.