Flexibility for homemade wig caps?

The flexibility will depend on the materials you use. If you go for the most common ones, like stretchy fabric and white PVA glue, you will end up with a quite flexible wig cap. I mostly make wigs for my OT 1:6 scale action figures nowadays, so I don't have to worry about eyelashes, you won't see a lot of damage to the painted surface but if you're like me and cover the whole head with the plastic wrap super tightly, you will see the fixative on the nose rubbing off and that area becoming shiny (due to the rubbing or friction). You can add layers to your inexpensive dollar store head, so that there's less difference in circumference, otherwise the wig might not fit. They are mostly flexible, but not stretchy, so 1cm will make a bit of difference when it comes to a "hard-cap," more so if the shapes of the head are different as well.

For an OT Hujoo Wings doll I painted myself, that does have lashes, I made a wig form using aluminum foil from the dollar store. I made it so that it was the exact shape and size as my Wings' head, and then I didn't have to worry about damage to his lashes or nose -- I mostly did it because I only use synthetic hair, which must be heat set, or it will stick out if cut too short. The aluminum foil form can be dunked in boiling water, so that's why I chose to use a form instead of directly sculpting the cap on my doll's head. It also helped prevent my Wings from getting any damage to his face up and lashes as well as possibly melting, and I can reuse the wig form whenever I want without any worry. So that's another option, if you don't trust the dollar store head. Good luck, whichever way you want to tackle your particular dilemma, I hope everything goes well for you. (:

 

I use Styrofoam balls when in need of a wig. They can even be shaped to match the shape of the head

 

Frankly, if that were likely to happen, somebody would already have had it happen and posted warnings about it.
It's a process that's been done for years by BJD owners and I've never heard mention of it. Ever.

Teddy

 

I've been making wigs for a few years now, and have no issue with saran wrap messing up faceups. Eyelash wise, I'm just careful to make sure there's extra saran wrap around the eyes so the lashes aren't touched.

Also, for flexible wig caps, I use fabric and 'mold builder' latex. It works great, though actually dries a little bigger. I end up adding a few more layers inside to 'shrink' it when I need to.

 

For what it's worth, I've made a couple dozen hardcap wigs at this point, using the method of molding the cap on the doll's head over a layer of saran wrap, and I have never, ever had any issue at all with the saran wrap damaging the faceup in any way. I genuinely don't think it's something you need to worry about.