I had an old door between two spaces and this door seems like a bay door , it was a half door (don't ask me why - I brought the house with it). In one space, I make a video game room and for my husband don't comply about the noise, I needed put a entire door. I didn't need a door to protect us, but we needed isolate the noise, then I decide make the other half part with cardboard and so I make my triforce door.
It's easy to make and if you need only isolation and privacy, it's work to you!
Step 1: Material
A lot of cardboard box and hot glue, remains of polycarbonate crystal plaque or acetate foil (a little).
First, take a measure were the door will stay.
To cut the cardboard, use a handsaw (used for PVC pipes).
Step 2: Cut
With the measure (see figure), cut 2 big pieces (A) or use fix little pieces united to other layer of cardboard (If you use this method, be careful for glue the pieces with borders disconnected.
Cut 2 strips with height door and 5 cm (can be altered) for the lateral parts. If you do not get a large strip, you can make like an explanation above and use little linked pieces.
Step 3: Assembly
Cut the pattern (if you desire) of triforce design (in the 2 sides) of the large pieces (A). Use hot glue to fix the 5 cm borders (B and C pieces) in the A piece. Fix stripes of 5 cm minus 2 X the thickness of cardboard inside the entire piece as show the figure. These intern strips do make a strong door. More strips, more resistant.
To the hinge, reinforce the local with wood or rigid plastic and use screw and fix inside with coil (do that until close with the other piece A.
Step 4: The End
If you will make the triforce window, put the polycarbonate or acetate in the inside and glue stripes over the poly or acetate exactly in the border of pattern cut (observe the figure) and cover (if you want) with poly or acetate again in the upper side (don't forget - discount the thickness of poly or acetate to make the right size of strips.
To my door to be a fashion door, I cover with a fine paper crease that here in my country (Brazil) is named "papel de seda" and stick with a mix of PVA glue and water. I paint with a mix of PVA glue, water and a gold powder (that here is named purpurina) to metal apareance.