I suppose you could build a space frame and just bond the panels to it

Depending on the car,you could still burn it,but without cooking the whole thing. Any removable panel and door can go on the fire,and you've then just got pillars,roof and rear quarters to blow-torch. Or surely,given the tubular trike contraptions about with half a Beetle/Skoda Estelle shell etc,that a carefully flamed shell with roll cage couldnt pass an engineers report..