What year is you TR6? I like the Miata seats that are available in the UK and Europe, like the ones pictured above, but the MK2 version with removable headrests are rarer than hens teeth in North America.
Two issues beside the use of the tonneau are the fact that the headrest on the commonly available Miata seat is one piece and rather too "chunky" looking for my taste, and that the modifications to the slider make one sit higher than with the stock seats. This is not a problem for some.
When folks started using Miata seats some 20 years ago they were relatively easy to find and were an economical alternative to replacing the foam and seat covers on the stock seats. I am not so sure that is the case anymore and if you have to replace the foam and the seat cover on a Miata seat, many of which are in bad shape by now, the costs will be similar.
I am currently rebuilding a set of late CR seats with removable headrests for my TR4A. I have ordered covers from John Skinner in black leather with white piping to maintain the TR4 look. I will add an inflatable lumbar support as well.