A little while ago, I asked Mark and Mary at Valley and Peak https://valleyandpeak.co.uk/ to make a draught curtain that I designed for my X-Mid. It weighs 31g and is made out of their bivy top material. It attaches with mitten hooks and lineloks with shock cord. It can be left in place and can go at either end. The corner near the trekking pole can either be pegged or attached to the trekking pole.
I’m really pleased with the way it’s turned out and the quality is top notch. There’s no price yet but V&P will probably put it into production soon. It’s a partial solution to the lack of on a solid inner, especially if you don’t want to buy the Ultra Bivy and want to use the mesh inner.