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!
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.
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.