Step 1:
Take an a4-size paper.
Prefold over this line.

Step 8:
Take another a4-size paper.
Prefold over these two lines.

Step 6:
Mountain-fold over these two lines.
Tape the edges on the left and right together, see arrows.

Step 13:
Roll a piece of tin foil around the bar, fix it with tape.

Step 12:
Use tape or glue to fix it.

Step 11:
Now it's easier to roll up the paper.

Step 10:
The four bars are made of these papers.
Valley-fold over the yellow lines, start with the line on the right.

Step 9:
Cut along the prefolded lines.

Step 5:
Valley-fold over this line.

Step 7:
The main part is done....

Step 3:
Make an inside reverse fold on each corner.

Step 2:
Prefold over these 4 lines.

Step 4:
Press the layers between the arrows against each other. Then lock them together by folding over the yellow lines.
Do the same on the other two corners.

Step 14:
Tape the four bars to the animal cage.
The bottom of each bar is behind the edge of the cage.

DuUUhn!

And this is a DIY-video by Joost Langeveld with instructions for making the animal cage: