I don’t have a definitive answer, but something to consider is this rule from the CCR. It requires that the bulkhead be sealed around the rear down bars so I wonder whether they would allow a big open hole. If you have a fuel cell they would likely be stricter.
cheers,
bruce
[quote]15.6.11 Rear Braces
The main hoop must have two (2) braces extending to the rear. The braces shall be attached as near as possible to the top of the main hoop, and no more than six (6) inches below the top. The braces must not contain any bends. There must be at least 30 degrees between the plane of the main hoop and the plane of the rear braces. The main hoop rear braces shall be installed to form no more than a one hundred five (105) degree angle or no less than a seventy-five (75) degree angle with the main hoop when viewed from the top. The main hoop braces may be mounted at the rear shock mounts or uspension pickup points (providing that the braces remain in compliance with all other sections of the CCR). They may go through any rear bulkheads provided the
bulkhead is sealed around the cage braces.[/quote]