I levelled up the chassis as best I could, by jacking the chassis up and placing it on axle stands with packers on top - my garage floor is far from level!
This took some time to get the chassis anywhere near level, but I wanted it to be right.
From underneath, with the rear wheels off, I could reach to drill up through the innermost mount holes with a right angled drill. I made a bushing to fit inside the 12.5mm hole in the chassis which allowed my 6mm drill bit to be central. Once these 2 holes were made, I placed some masking tape across the width of the upper boot floor and drew on a straight line between the 2. I then used my cardboard template to mark the other 4 holes that were required.
Once I was happy with the position, I drilled the 6 holes in the upper boot floor - 4 @ 60mm, and 2 @ 50mm to allow plenty of clearance for the base of the roll hoops. These lined up well with no problems.
Firstly I made a measurement device so that I could detect the hole centre, and how far off I was. I used a nylon M10 top hap washer, slightly padded out with a rotation of tape (taking care to ensure the tape ends lined up) and an AutoCad created 1mm Grid with centre line. This was stuck to the washer with double sided tape. Next the awkward bit......
I placed masking tape onto the underside of the rear bodyshell, so that I could project upwards a plumb bob mark. This involved climbing in the boot, twisting, laying on my side / back / front to try to see what the plumb bob was doing. I held the plumb bob just above the marker and using the thumb nail of one hand, held the string to the underside of the bodyshell and marked with the other hand the central point. I did this for all 6 holes, and then double checked them.
They all seemed not too far away.....
I then drilled a 2mm hole up through the bodyshell at each mark to enable the plumb bob string to pass through to get a real reading.....
There was only 1 hole that was around 3mm out! I drilled a new hole just to the side, and then it was spot on.
Now came the twitchy bit. Rather than go for the 60mm holes and 50mm holes which would give the required 5mm gap all around the roll hoops, I went for 50mm for the front holes and 38mm for the rear (the exact diameter of the roll hoops). I drilled the 50mm holes first and opened them up very slightly with a drum sander so the legs could just go through. I tried the first hoop in and it fitted!
I then drilled the hole for the rear leg (38mm) which again, after being opened up a little, fitted nicely. Or so I thought.....
After I had fitted both roll hoops, I tried to tighten them down. I realised that they wanted to pull forward slightly. I had to remove them and elongate the holes towards the front of the car a few times until the hoops fitted. This possibly means that the chassis wasn't perfectly level prior to the drilling!
Not to worry, once the required 5mm gap was drawn all around the bars, the biggest gap I had was 6mm. Well within tolerance of the large rubber grommet that gets fitted along with the aluminium cover. I still have to fill the hoops with expanding foam to help deaden the noise, and I probably won't finally fit the hoops until the boot carpet has been fitted.
All in all a job well done that was probably the job that I feared the most!