This Google Cardboard compatible viewer is capable of supporting of phones up to 4.8 inches in size and is a perfect fit for a Samsung S3 with a case; 5.0 should still be doable.
It's made up of multiple parts for easy modification and relies on M3 bolts to keep it all together. I do suggest adding some padding to keep it all nice and comfortable if you're going to use a strap.
This current version features 38mm lenses which I "borrowed" from my old Oculus Rift DK1 but I'll make a version for use with the official Cardboard sized lenses as soon as it arrives.
ADDITION: Now includes an improved visor. It uses almost double the amount of plastic but allows for easy adding of padding and has a top strap holder.
12x 16mm M3 bolts
1x 10 mm M3 bolt
2x 38mm lenses with a focal length of 38.5-40mm