I think from memory the box was 50x50mm and the plates were 10x75mm cut to 200mm. They were bolted together using M10 studding with nuts welded on as it was cheaper than bolts. Uprights were round tube to suit the wheels. The clamping effect from the plates on the box section worked better than I thought and the bolts did not need to be murdered up tight to hold firmly.
The car on the dolly in that photo is currently up for sale on Ebay if you search Jensen CV8........
