Firstly it is important to ensure the car is the right shape. If the car has been rebuilt and especially if the floor pans and sills have been replaced it is easier for the body shell to end up longer than standard, than it is for it to be shorter.  The chassis on TR4A-6 are notoriously weak and can bend in the middle which will result in the rear of the cockpit moving away from the top of the windscreen. Check your car against similar cars of good provenance to ensure your cockpit dimensions are correct. For reference on a car with a backlight frame to accommodate a surrey top, we know the dimension between the holes in the top of the windscreen frame for the surrey spider frame and the holes in the backlight frame to accommodate the spigots of the rear frame is 760mm.

 

Covers are made from hooding material which can either be vinyl or mohair. These materials when new are relaxed and must be significantly stretched when being fitted to ensure a taut fit in service. This stretching will require significant force which must be aided by ensuring the cover is not just warm but really quite hot, equivalent to a very hot summer's day in direct sunlight so that the cover becomes a floppy bag. In this way the cover will be taut at speed and on seriously hot days and not revert to a floppy bag. This level can be achieved with a hot air gun, hair dryers or a fan heater, ensuring all safety requirements are met. It will be surprising how far the material will stretch and how this amount of stretching when the cover is fitted will result in neat installation with no creases in the material.