Let's just get to the bottom of this...
Any wig that FITS can be glueless and I mean this is the most direct way. I have worked with every type of wig and when I say type of wig I mean I started working with u-part wigs 8 years ago, then closure wigs and then frontal wigs and then full lace.
The key to glueless wigs is the fit. If it fits properly it should be able to clip on to your head or sit on your head without adjustments.
A U-part wig can be sewn on if the grid of the wig fits the roundness of your head and around the area that you will have leave out. It can be sewn down or clipped on as long as the wig is made to fit.
With closure wigs, frontal wigs and full lace wigs if the unit fits the circumference when your hair is flat (meaning braided down or bald) you should be able to slide it on without using any glue, gel, spray etc.
There are things that can be put in place to help the unit feel more secure when its glueless like combs, elastic bands and adjustable straps. These are all things that do not require glue to secure your wig.
Some wigs made on certain caps do stretch if worn daily but they do shrink back when shampooed and dried.
Always know your head size prior to placing an order for a wig. If the wig maker says "one size fits all" for a glueless, IT'S A LIE, SIS! (I'm crying laughing) No seriously they cannot guarantee the fit and most people have not even done a consultation for you so how would they know it would fit?
Wigs made custom can range from 1k - 10k depending on the route taken to make the perfect fitted wig. Ventilated wigs are fully customized and used on movie sets and those wigs are molded to fit just for you. Right now we only customize hairlines with ventilation because of the amount of time it truly takes.
If you experience sliding I recommend a wig grip especially if you are not wearing braids under your units. The braids help prevent sliding because the wig has something under it. When the hair is not properly flattened (like all my girls that pull their hair into a pony tail because they don't know how to braid) the unit tends to slip more because the pony tail adjust the size.