Wooden blocks were used and once airborne flaps were opened up fully to let the blocks fall out then flaps closed for normal flight.Its what they did the the MkVc Trops that were flown to Malta off carriers.
Filtered air kept the piston rings alive..Cool oil kept the Crankshaft and cam bearings alive..Also in the Tropics there were mods to the fuel system as well as tank pressurization stopped vapor locks and starvation problems..
Yes the smooth cowl was only in use for a short amount of time...Coral dust was found to be more abrasive than normal dust off memory and caused a lot of drama.The Australian Archives does have the Australian Smooth cowl GA drawing and offset measurements to create one.Was a straight bolt up job...
It wasnt just the filter but the Tropical Oil tank was longer and held more oil to keep the Merlin cooler in Tropical climates.It gives the cowl a Pigeon breast appearance.Same as the PR Spits.
79 Sqd RAAF changed to a locally manufactured smooth bottom cowling when it was escorting B24,s out of Townsville.
https://www.alamy.com/stock-photo-79-sqn-raaf-spitfire-awm-p02874033-171086403.html
Bottom part is pretty basic,almost like a WW1 throttle.The handle itself holds the gun selector and in front of that the trigger..The diam of the main quadrant is 185mm.Im having drama getting into proper CAD layout so it reads flat..