You can get standard Tonneau covers for Hearld and Vittesse convertibles, here is a link to one company that makes them but I'm sure most of the
Triumph Traders would also be able to supply:-
http://www.prestigeautotrim.com/uk/hoods/herald_ho...
I used to have a Spitfire without headrests with a stanbdard flat Tonneau Cover and later when I fitted headrests and a Roll over bar the Tonneau
didn't fit anymore. I took it along to a local coach trimmer and he added some headrest bulges and slots with a velcro arrangement to fit around the
rollover bar, this was a nice neat arrangement and I used it quite a bit for top down motoring on cold but sunny days.
I'd suggest it would be easier (and probably cheaper) to buy a standard tonneau for your car and have it adapted for your headrests by a local coach
trimmer rather than make one up from scratch from you piece of material.