kev, I'd be interested in trying the Super Seven pad w/o the headrest pad. Based on my experience with beaded seat pads on motorcycles and the Slingshot, I'd probably use one year-around since the Slingshot seat is in a cockpit and not fully exposed to the air.
I understand those folks wanting delivery at MV to save some coin, but I seriously doubt the Dutch seller would be able to get a shipment over here in time.
Anyway, keep me in mind if you decide to get some. I can either take two or just one, depending on how hard it is to get them. If you decide to not pursue this, let me know and I may try ordering them direct.