Finding ways to build rolling stock that fit in with the Mamod style is something that I like to do when not working on loco ideas. One of the simplest approaches is to utilise existing parts from O-Gauge Hornby rolling stock. Various wagons lend themselves to such modification. My favourites are the Hornby No 1 Rotary Tipping Wagon and the No. 1 Crane Car. You can fit your donor parts directly onto the Mamod/MSS flat bed wagons to build some rolling stock that looks in-keeping with the Mamod SL scale and overall appearance.
MSS have also bought out their own wooden bodied coal wagon. This is sold in plain wooden body or grey. These provide a great basis for further modification and different load types.