John, as an alternative you could look at the BMW Z3 deflector.
I had one fitted to my DHC, although I did not have a roll bar as well. The Z3 unit needs a modification to the mounting brackets to make it sit upright in the car, but as the brackets are fixed to the rear bulkhead (screws through the trim panel) you have a certain degree of flexibility over the exact positioning which should allow the deflector to sit in front of the roll bar or lower down to sit under the roll bar. The deflector can also be set at different angles from the vertical once installed.
The deflector is black mesh & looks so much better than the clear acrylic IMHO. I was pleased with the reduction in wind turbulence. Once installed it is very easy to remove if needed, just leaving the brackets in place, or it will fold flat onto the rear deck. Being BMW it comes with a decent storage bag & even bungs for the mounting brackets to stop water getting in when the deflector is not in place!
If you look at this you want the early version for Z3 WITHOUT the roll over hoops (pre 1999 I think). They are often on fleabay, under £100 should get you a new one.
Sorry but don't have my DHC any more (sob) so can't measure exactly for you, but I do have the cloth storage bag (forgot to give it to new owner) so I could measure that for you if any help!
Tony
<font size="1">1981 2L FHC Cavalry Blue
1980 3.5L V8 DHC Jaguar Regency Red - sadly sold!</font id="size1">