I've been asked before for pictures of how I design the inside of my fursuit heads, and so here is an example (The picture is over a month old, but I just got around to putting it up) of my current method.

The construction of the frame/head-mesh is entirely based on foamies (thin sheets of dense, pliable, close-celled foam) which I cut and glued together with hot-glue, and often laminate for additional strength (Especially on the snout and hinge (yes, the mouth moves). The resulting head is quite tough; even though it can get squished/deflated if something heavy is placed on top of it, I just need to stick my hand in, and it pops right back into its original shape. I've found that heads of this design can take a surprising amount of abuse. A bonus feature of the elastic nature of the foamies is that the reinforced jaw hinge also acts as a great spring, allowing the mouth to open and close with no additional parts, and - if laminated - with a decent amount of strength (enough that you can easily mount relatively heavy teeth, tongue, etc inside the mouth without worrying)

I actually don't need to use a zipper or anything to get my head in; once the frame is furred, I simply slit the back strap under the fur, allowing the fur's stretch (Which is far more than sufficient; it can be done with fairly stiff fabrics) to accommodate the extra width of my head.

The resulting construction is both light and tight-fitting, which means that it doesn't trap a lot of air between your head and the interior of the fursuit head and isn't as much of an effort to wear (being light), which means that it stays cooler than designs with thick layers of foam or lots of interior space.

This particular head was made for a prototype/experimental fursuit (Which I normally do not show off, as they are often rather unfinished, designed simply to test a handful of concepts) which I decided at around this time to convert to a production-grade suit for myself of an umbreon.