The wheels are from a generic cement mixer toy I found in a bin at a thrift store. Everything else is built from sheet styrene from a hobby shop.
The front of the trailer has pegs like the fire truck/axe to connect to the cab's back, and the feet slide into a recess on the underside:

The inside of the trailer is limited due to the foot notch, but there are a few deluxe figures that fit. Blurr, Jazz, and Bumblebee can all go for rides in the trailer. I tried Lockdown too, but his wheel spikes make him slightly too wide.


With the trailer attached, Prime's alt mode is the same size as leader-class Ultra Magnus. It doesn't split in half or have a repair bay or TFA Roller, sadly. I'd love to rapid prototype the model and airbrush it, but then I'd be out of money for collecting. In the mean time, this looks very nice with Prime on my shelf!
Edit: Fixed broken URLS