Andy,
The positioning of the screen is determined by the profile of the screen and scuttle on the R500 screen this brings the front edge of the screen 70mm from the front edge of the scuttle. I would caution about putting gloop under the screen and pop riveting in place. Shure it will be fixed good and proper but you will never be able to get it off without wrecking the paint job. Why not use 4 X 5mm nuts and bolts with washers and then run a thin bead of clear Silicon along the leading edge when its fixed, this will keep the debris and water from finding its way under the screen and will ensure you can remove it if required. Tried the car today and enjoyed the experience, took me back to my early motor cycling days plus the car feels much more lively after dumping all that weight and having 230 BHP 162 lb ft of K under my right foot.