Adding a trailer car to Kernow/Bachmann DEMU

Hi all.  I have a plan to adapt a Two Car DEMU into a three car.. I have two Kernow DEMU's and I would like one to be three cars, I am looking for the correct style, seating arrangement trailer car to adapt. Which will need suitable Bogies with the correct drawbar fittings ( & Drawbar ) to continue the flow of current throughout the unit ( with additional wiring/ lighting strip ) First of all, can anyone tell me a suitable. (BR Green )
Trailer that would fit the bill?   all the best. Kevin