you may well have this sorted now but i made 1 a while back for my project:










from memory the main T`s at each end are 80x80x6mm thick box..

the pins are 40mm bar..

the box section actually fitted to the shell is 40x60x3mm thick (i think)

the wheels came from 1 of those BIG wheelie bins found in industrial estates.. they use a 13mm nut and bolt and can be removed very quickly in the middle of the night - if you struggle to find somewhere that sell them hahahah

the bar under the car is just 50x50x3 box section... the locking disc was done at a local laser cutters, 10mm steel and they charged £20 to cut it with all the locating holes..