Reddit mentions of Antrader Corner Protector 24pcs Metal Box Edge Safety Guard Bronze Tone 1" x 1" x 0.35"

So what I've done for my 121x80cm (3000pc) puzzle is the following (much like u/Excipa):

1. Flip the puzzle over (time consuming but necessary) I use two pieces of something like foamboard, as rigid as possible, put a small square of the puzzle on one, then sandwich on top and flip it. Put it all together upside down.
   
   
2. Lay down contact paper on the back and roll it out (the stickier paper the better, the puzzle-specific stuff is probably extra sticky), if you get air bubbles that you can get rid of, pierce or slit the paper and roll the bubble down. Trim excess.
   
   
3. Find a stuff backboard, I used corrugated plastic. Cur to size, I used gorilla tape to join pieces. Use a spray glue or double sided tape(lots) to join the back of the contact paper to the sized backing sheet.
   
   
4. Then I cut a frame out of something like this and stained it. (You could get an actual picture framing board) Then used edge protectors to secure the frame pieces together.
   
   
5. I got a piece of acrylic glass to protect the puzzle's front, which is much less expensive, less heavy, and less breakable, than a custom piece of real glass. Not to mention easier to cut to size on my own.
   
   
6. I used gorilla tape to attach the frame and backing board together (probably better ways to do this using actual picture frame hardware). And I used a couple of the big 16lb command strips on the back of the backing board to hang it on the wall.