I was thinking of either making one, or having one made for me.

Could I take a piece of tubing, weld some plates on the end of them + then bolt the plates to the the rear shock towers where the rear seat went to?

 

The rear shock towers would not be a good idea as the only thing holding the bars. The nuts on the towers themselves are small and only are to be torqued around 30ftlb. I would not trust them solely with the forces during a car accident.

If you want to use the shock towers, also make sure to use other anchor points such as the lower seatbelt bolt or something similar that will hold your weight in case of a bad crash....

 

Okay. I'll just get an actual roll/harness bar then.