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Really impressed with the pictures of the Interceptor! Like very much what I see.
You comment that it is difficult to do with the windows down...Have you thought to build it as a two piece on a 'sprew' so that you have base and interior, fitting into the body shell as a separate piece?
May need to reduce the dimensions of the seats and wheel etc but not by much and the body shell can be make fairly fine as, once it is glued to the floor section, they will be very strong.
Would love to see you succeed at something like this. All the best