UPDATE 9/20/23:

Added Lighted skull version due to many requests! Skull with eyes lighted version works for lighting or not.

Changed Jaw bottom to lay flat on build plate for risk free, no support printing.

Brain bottom edges improved for no support printing and cleaner outlines.

The eyeposts ( and all other parts) remain the same. 4mm tubes running internally from eyepost mount holes to bottom rear of skull which has a flat shelf (wide enough for 2xAA battery back or other solution) to glue or velcro batteries and switch. A battery pack WILL protrude from skull geometry, as I didn't want to reduce candy capacity to accomodate). 5mm LEDs fit through the eyeposts. I recommend twisting +/- wires to give rigidity when running through, and heatshrink (or other solution) wire ends together so the slide through better. I prefer to adjust LED inside the middle or rear of eyepost to illuminate the socket as much as eye iris, and then hot glue bottom wire exit hole (NOT the eye socket end) to fix that distance.

Remixed from my previous large lighted eyes skull https://www.thingiverse.com/thing:5506102

which was a massive remix from Makerbot Human skull https://www.thingiverse.com/thing:622390

Brain extensively remixed from Wigglememore https://www.printables.com/model/112498-real-human-brain-from-mri-scan/files

Skull sits fine and level without jaw if you want to skip that.

Skull cap not necessary if you desire brain to be exposed full time

Printing brain with flesh / skin / beige / light grey colored filament should make it POP, especially if not painting skull.

3 of the pictures are the Skull, skull cap with supports still attached so you see how they finished.

Brain: Recommend 3 walls with .4mm nozzle (or .6mm nozzle and 2 walls).
Shortest layer heights you're comfortable with, to reduce island effect. .16 or less looks good.
No supports
15%+ infill to support top.

FYI: If you ordered or used Hatchbox ABS Beige for brain, it does NOT vapor smooth well for me! (nor did Hatchbox Black). Brushing acetone on the brain DID yield a WET (shiny) looking finish. So I suggest lightly brushing with acetone for easier and faster method to achieve a GROSS creepy effect :-)
OR print in flesh/beige PLA, and coat with XTC-3D or other resin. It prints easier and looks GREAT slimy.

SKULL: Versions with eyes (lighting optional) AND without eye post cavities (in case you don't desire to spook it up with eyes and craft).
Sliced with 1.2 wall thickness. So 3 walls for .4mm nozzle, but I prefer .6mm nozzle for large prints and 2 walls (acceptable given no small features), whacking a lot of time off.
Modeled for support overhang at 60 degrees. Lower setting results in more unnecessary filament and time. TREE SUPPORT strongly recommended over normal.
Make sure to print upside down and that rim is set on bed all around.
4 bottom and top layers. Support Blockers where wire channel starts at back of eye socket.
Using 3 cutouts of thin nonslip material adhered to brain shelf will help eliminate plasticky sound when placing and moving around.

Skull Eyes: If you choose the eyed version (cavities in sockets for posts), you can use Eye Posts file for protecting those holes while painting.
Eye Ring Posts fit in these holes and you can mount the Eyes file to those. They are hollow and fit 5mm LEDs. Pictured version uses 26mm eyes from Amazon. https://a.co/d/8NARPsz and only the BROWN set shows iris illumination. These look just the same for $5 less https://a.co/d/ezhA97u and I just tested: Many more eyes, but none of them show iris illumination. So if that's important to you, go with the first set. Pricey for the brown eyes, but worthwhile for many years of spooky.

Alternatively, use Eye Blanks and hot glue your eyes to them for off center pointing eyes.
Fun if he looks left, right, up, or is even cross-eyed! Eye Tubes added in case you install LEDs before painting and want to protect during painting, or print clear and use instead of ring posts for your custom eye concoction.

Skull cap: Optional, but my tribe insisted I make it work with brain underneath.
Wedges print separately (they're identical), to be glued on underside on the 4 flat pads of Cap print so it stays in place on skull.
Wedges provide just enough clearance from brain underneath (or will center it when placing) and cap will rest flush on skull (as pictured).
Wedges are not a totally snug fit, so cap will shift around a little when carrying your scary sugar dispenser / receiver bucket.
Same Layer height guidance as brain.
Set overhang 60 or higher and use Support Interface (or you may have a fight removing).
70 should be good balance of topside print success and easiest removal.
Be PATIENT, slow and gentle. It's a large supported area and will take finesse to remove.
Low end of filament temp range will also help it not fuse to model.

Jaw bottom rests on build plate as opened in file. 2 walls, No supports, 0.4mm nozzle. 
Check Small Feature box in Cura experimental or slow print when getting to the teeth for better quality.
Recommend lightly sanding/filing square edges of the 2 mounting pegs to slip into skull better.

PLEASE leave comments to ask questions in advance if you please, and alert me to general problems (good luck finding one :-), and review like and dislike… alike LOL