Get a lot of cardboard... then just go for it. Leave the complicated cuts( i.e. lens holes) until you've made all of your mistakes on the easy ones!

Design 1:

Don't make the viewing tube too big... you want the phone to be a bit bigger than the hole.

Easy to adjust lens distance to phone - lenses are mounted on separate piece of just the right height and folded to width for stability.

Design 2:

Requires a phone case that straddles the back cardboard edge.

You can't adjust the distance of the lenses to the screen afterwards!

Trickier to make robust. I was lucky my phone case squeezes in. Otherwise you might need to put a lid on it to keep it solidly square.

Tip: Use heat on bridge of frame and stretch to adjust distance between lenses to match your phone exactly.

Design 3: VR2Thin - See SVG file for some help if you want to try building one yourself.

Cardboard isn't firm enough... use woodboard ( from a clipboard ) or 2-3mm plastic. Thin plastic for the wing, back and top pieces.

NOTE: Focal point of lenses need to be 70mm ( i.e. these "zoomer" Dollar store lenses ) or modify design accordingly.