A 'true' tonneau cover would be nice, but I don't see how it could be successfully secured for high-speed driving. My XK120 and 150 had tonneau's and they worked quite well (no inside flapping that I can remember). When I was dating my Lady-to-be, we were driving from Boulder to Denver in the XK150 with the top down. It started to snow and my Lady started to complain that she was getting cold. Not a problem, pulled over and quickly put up the passenger side tonneau and she laid under it. A plus was the heat from the heater actually could be felt (I always tell people that an XK heater only kept itself warm). Please note that the top on XK150 was a cleverly designed piece meant to frustrate any driver in a hurry to put it up.
The Jags used a combination of treaded studs and flat snaps to secure the cover. Magnets would be tricky as no guarantee that they would hold at speed. Making the tonneau to fit over the head rests isn't that big of a challenge (the driver's side in the XKs was fitted for the steering wheel), but would increase cost over a flat tonneau.
In the tonneau thread that was linked into this thread, the slope is important. If the dash is lower than the body behind the seats, that is a problem with water flow. Note that any tonneau would need to be removed before the top could be raised (nothing new as it was the same with the Jags).
In all, I do miss having a tonneau. I have parked La Bęte leaving the top down but had to remove leaves and other bric-ŕ-brac afterwards. That is something you don't do if you have a tonneau. Chances of intermediate light sprinkles/showers are handled with a tonneau. As also pointed out in the other thread, the speed/ease of putting the top up does mitigate the need for a tonneau. I do think about the wear-n-tear on the top system given the number of times I operate the top (daily on most days in spring and fall, less in summer and winter).
I do feel a tonneau looks sharp on a sports car. It is what I grew up with and no OTS was complete without one. In my eyes, La Bęte looks a bit naked without one.