My brother Steve gave me a Google Cardboard VR viewer as a stocking stuffer for Yule, because yes, we live in a time when virtual reality hardware can count as an inexpensive stocking stuffer. I haven't been able to find exactly the model he gave me, but if you do some looking around on sites like Amazon and Aliexpress, you can find working versions for as little as $5.00 CAD.
I've got a review of the experience on my main blog, but this post is about what I did to harden the viewer a bit. The Cardboard viewer Steve gave me is much nicer than the basics — the cardboard is covered in some retro-looking red textured stuff, it has a head strap and an NFC tag, and there is an array of suction cups on the front flap to hold the phone in place — but in practice the viewer turned out to be annoying because of a few design issues. You can see in the photo above that I already bent the top flap to accommodate my Note 2, which is fine. I acknowledge it is a huge phone, and I've had to mod things before to get it to fit them. But I was also having trouble with the viewer starting to fall apart as I used it with Cardboard apps. The side with the magnet switch on it was gummed so it could stick to the layer underneath and hold the whole viewer together, but it had a bad habit of peeling off, especially when I was using the head strap. The divider piece which ensures each of your eyes are fed separate screens of information kept popping out of place and going crooked, and the magnetic switch only worked about 20% of the time.
So I did what all good crafters do when faced with wobbly bits of assembled cardboard and plastic: I got out my glue gun.
I glued all the tabs into their slots, both for the eye pieces and the divider piece, and I glued the flap with the magnet switch shut. Then I glued the top seam of the flap to the top of the viewer for good measure (far side of photo below):
I discovered after doing this that the bottom tab of the divider was still escaping its slot, so I eased it back into place and then glued along both sides of it. While I was at it, I put glue over the nose rest in several layers, finishing with a piece of sponge. That was to make the nose rest more comfortable when using the head strap. It's still uncomfortable, but it's less uncomfortable than it was before. The glue isn't visible when the viewer is being used.
You can see the hole I cut out of the front flap/viewer bottom in the lower right of the above photo. That's to keep the volume rocker switch on the phone from getting accidentally pressed while it's in the viewer.
There was going to be one more mod, which was to add a simple conductive touch switch made of aluminum foil, cardboard, and sponge as described in this Reddit thread. However, I discovered that now that the shape of the viewer is stabilised by the glue, the magnetic switch works much better. It works around 80-85% of the time instead of 20% — often enough that it's not too annoying anymore.
A lot of the simpler viewers have instructions to hold them together with an elastic band. I'd recommend taking the time to glue to avoid the different parts of the viewer shifting out of alignment. Yes, it will make your viewer look like it's covered in strips of snot and/or ectoplasm, but face it — anyone using any kind of VR viewer looks dorky anyhow, You may as well make sure the experience inside the viewer is worth it.