Its pretty obvious that except for very casual viewing, a headstrap/headmount is a must.

Ive done it 3 ways so far.

The simplest I used existing slit holes on the side to pull elastic banding (I got mine from a 3 for $3 pairs of shop safety goggles from a local supply store. (don't worry I plan to use the sides of the now strapless goggles to make protective sides for my glasses when in the shop) just pulled it through the holes and knotted it off so it could not slip past

The 2nd technique, I used a large square of self adhesive velcro. and added it to the top of the cardboard, and onto the bottom of the bill of a baseball hat. that works pretty well., although you can have the cardboard kinda flap up and down banging into your cheeks unless you do it plus the headband.

3rd technique, I picked up some snap setting pliers, and snaps, and tried out crimping on snaps on elastic (works great) and making snap points on cardboard, i also added a snap at the top of the velcro flap to the middle back of the around the head band, which nicely helps support the weight of the phone.

The snaps may sound like they may come unsnapped. but snaps are very resistant to pulling open at 90degrees from their normal snap opening/closing. so really they are as secure as something riveted through cardboard really can be. (and the phone is padded by cardboard if it does fall) (I will post another instructables about setting snaps in cardboard.