Hollow Knight Helmet

Finished Hollow Knight helmet

Light-weight, accurate costume head of the main character in the game, Hollow Knight.

My son and I are big fans of the game Hollow Knight, which features an awesomely adorable player character with a mask-like head. I had been watching a lot of Bill Doran and Ted Smith videos on how to build costumes out of EVA foam, and had been itching to try and build something myself. So, here’s the result.

The Process

Ultimately, this was going to be a big, round, hollow shape. To start, I would create a three-dimensional form from which I could make a flat pattern. Based on my son’s head dimensions, I went into Photoshop and made a version of the Knight’s head that would fit comfortably on him. I had to move the eyes a bit so he could see.

Carving insulation foam

I got big sheet of foam insulation, cut it up, and spray-glued a stack together. I printed my mockup at scale, and traced the shape and eyes onto the pink foam. Then I cut, sanded, cut, and sanded until I was happy with the shape. Then I printed the side mockup and did it again on the side, then the top. Then more sanding.

Next, I covered the finished form in masking tape (only needed to do half since it’s perfectly symmetrical), and marked where I thought the seams should be, adding registration marks so everything would line up.

Making patterns with masking tape

Carefully, I cut the masking tape off the form, and laid the pieces on some poster board, adding darts when necessary for it to lay flat. Then I pinned them so they wouldn’t move and traced ’em and cut them out.

Completed patterns

Next, I traced these patterns onto big pieces of floor mat foam (EVA foam) and cut them out with a SHARP utility knife. Foam dulls blades shockingly quickly, so I had a knife sharpener handy (thanks, Foamsmith!)

EVA foam pieces

Next I used my heat gun (basically a high-powered hair dryer) to gently sculpt these pieces into rounder shapes. Heating this foam makes icky fumes. I wore a respirator mask with charcoal filters and I made sure the room I worked in was well-ventilated.

The universally-beloved glue for putting this stuff together is Barge contact cement. It’s amazing stuff, but it is smelly and toxic. I wore my respirator again, and made sure I had good airflow so I didn’t stink up the house.

This is when I ran into a problem. Apparently, my 6-sided pattern has some inherent tension in it. The edges just want to pull themselves apart. Also, I used too much cement – less is more when it comes to that stuff.


I just called it “practice” and started over with a new pattern design. Fortunately, I could still use the same pink foam form.

New EVA foam pieces

Glued this bad boy together and it looked great!

Assembled helmet

Next were the horns. Originally, tried carving them out of the pink insulation foam and then coating them in layers of wood glue. I thought this would be lighter than just using the floor foam. They turned out okay, but the shell was quite brittle, and it was difficult to attach anything to the base of them – how would I affix them to the rest of the helmet?

I ended up redoing them using stacks of floor mat foam glued together and carved out. Turns out this weighs the same. I attached neodymium magnets to the horns and to the helmet and they stuck great!

Then I did a thorough sanding and heat-sealing, and filled the remaining cracks and dings with Quick-Seal.

Sanded, sealed, patched, with horns

The inside was finished with some pieces of upholstery foam, hot glue and trial and error. Painted with about 6 layers of white Plasti-dip spray paint. I experimented with layers of black tool to make the eyes matte black and opaque from the outside, but still see-through from the inside.