Blanket modification query

Sounds like adding a wind shirt over a thin puffy – not much.  That’s around 3oz of fabric.  You’d be way warmer at half that weight with an insulated balaclava.

Yes I have also considered adding a hood rather than the simple slit.
I also contemplated adding a thin layer of APEX as well, weight and bulk being less of an issue for base camping. If using just for the two of us no hood and no slit tho.

You could use a smaller piece of apex, maybe just where it would contact your upper torso as a poncho.  Adding some means to hold it shut as a poncho, to cut down on drafts, would also be weight well spent.

Thinking it thru and with comments in a separate thread I don’t think I’ll do it on the blanket I have ordered. It is a little large
I do think I’ll look further into it if I save enough to buy a smaller and lighter version with the taper towards the feet and I would include a hood
If I was still into Alpine climbing I would definitely modify with a shell layer in UL Robic tho as a 6 foot wide blanket can cover 3 people at a pinch and is luxurious for two