Motorsports Yearbook - aka The Blue Book - THE RULES.
T10.10.1. Not more than two spare wheels may be carried externally, and these must be fitted as close as possible to the
bodywork (excluding bumpers).
T10.10.4. The spare wheel(s) or mounting(s) may not extend rearwards more that 200mm from a vertical line passing through the rearmost part of the original bodywork (excluding bumpers), or more than 100mm beyond the rearmost part of the original bumper position for the model of vehicle, whichever creates the lesser protrusion.
Such a spare wheel rack would still be legal in Class 3, When you build the rack, give consideration to rear vision:-
T9.8.5. Rearward vision whilst properly seated within the confines of the vehicle’s bodywork or ROPS must be possible. Vehicles with solid rear bulkheads or an obstructed view along the centreline of the longitudinal axis must be fitted with either external rear view mirrors or a reversing camera. Mirrors and/or cameras must be operational during competition.