I welded the the bits where the top bars connect and drilled the holes for the bolts.
I purchased these Linear Guide Rails from Hong Kong. There were cheaper ones from china however their shipping date was estimated for 2 months.
The next step for me is to fit the frame to the car and start working on the car-cabin interface.
I can also start fitting plywood to the side walls only.
I need to purchase a circular saw to achieve this task.