Thanks Stuart. Sadly the insert sections are NCA (not currently available?). But I guess I can make something myself. Just need the correct measurements. I will try to find someone with a TR4a nearby to look and take measurements.
My brother owns the TR4 in the background of the picture. This has a TR4a window surround frame as I found out (with the TR4 aluminium top cover), it has the threaded holes for taking the frame catches. So the plan is to swap the surround over and put a TR4a top cover on.
Apart from the B post, I think I have the main parts to convert: a TR4a window frame, TR4a window top cover, TR4a top frame with front windscreen metal plate, TR4a catches, TR6 softtop with the rear steel support to be bolted to the rear deck. I found out this uses the holes already conveniently there in the TR4 deck.
Of course this is not all. The softtop needs to be fixed (glued and ) to the front metal on the frame. Behind the door window I need to make a support for the softtop side buttons. Also the Velcro at softtop, above the door window tops needs something to put it against to on the frame.
I am not too concerned about the forward deck extensions. The top will not fold as low as on original cars - these supposedly have an indent on the rear wheel arch but I think I can live with that.