While sandwiched plates are accepted on unibody cars, I was HIGHLY discouraged from doing so.
The Midget is a unibody construction, but what was recommended to me was to use rectangular tubing along the inside of the rockers. This more evenly distributes any impact over a larger area, and lets you tie the front and rear together more effectively.
Here's an early sketch done by Skeeter, my cage builder. There are more bars now, but this gives you an idea as to where you might consider starting.
Skeeter described the finished work as "an 8 second cage in a 16 second car".
Does an X-frame car use isolated body mounts?