I am attaching a pic showing the AL channels I glued to the vertical edges of the side curtains which stiffen them and keep them from rattling. The curtains are not stock and came from NICAL Engineering in the UK along with the hardtop, but the concept can be used on any side curtain.
BTW I had the side curtain brackets made by a machinist and I tapped the stock door sockets so I can screw the curtains to the door, providing a rigid mount and preventing the curtains from flopping around whether the door is open or closed. The seal both to the door, the hardtop and at the windshield stantion is pretty water tight (for a Healey) when the doors are closed and the curtains do not pop out at speed.