Piston Provost ill-fitting canopy
From my year at Cranwell (actually, in context, Barkston Heath and Spitalgate Sep 1959 - June 1960) I remember nothing of air sickness on my course, nor of drafty cockpits or fumes in the cockpit. If Haraka says so, there must have been a problem at other schools and we might have been lucky.
I cannot work out how the vented fuel should get back to the cockpit, as I seem to remember that the vent was at the very tail of the aircraft. One can imagine that air might creep back up the fuselage in the surface environment, but inverted in a slow roll is when there is positive pressure over (under) the canopy. The problem must then come from the engine. In normal flight the canopy ought to be evacuating fumes etc through the venturi effect.
Could it be that the fumes (let us hope not fuel) are coming through the firewall ? I think we have discussed elsewhere the loss of an admired PP owner when his engine fire broke through the firewall.