FYI, a single non-triangulated hoop won't cut it.
The SCCA allows autocross in an open car without a rollbar. They require a helmet only.
DE events with the SCCA or PCA require 5 or 6 point harness in an open car. Also the rollbar must be triangulated, and at least 2 inches clearance to the top of the helmet. They also require arm restraints for an open car. They also have the "broomstick" rule where they place a broomstick on the windshield and rollbar and the stick must clear the helmet. It matters not that the Speester windshield is not structural in any way.
Most times they both require long sleeves and pants, but that gets suspended on really hot days.
Make sure you empty EVERYTHING out of your car and frunk, nothing loose is allowed.