I recently made these for my boys and they absolutely love them, very easy to make and cheap
Pringle tube (or tube of similar size)
Something to poke a hole in the tube with.
Cut the metal base off the pringle tube and start that end, remove the plastic lid also.
Poke a hole in the tube just under the width of your rope, I used 18mm rope for mine.
Poke a length of string through the hole
and tie on the end of your rope. If you use damp string it will tighten more as it dries. cut the ends close to the knot. Start at the cut end of the tube.
Do the same on the opposite side to hold the rope in place.
Then wind the rope around the tube tightly pushing it up with your thumbs so there are no gaps,
Once at the other end you can use the rim to keep the rope tight. Doing it the other way its difficult as the rope slips off easily.
Once you have all the rope on tightly poke another hole and use string to tie it onto the tube.
Also secure the opposite side to stop the rope slipping over the lip.
Cut off the excess rope and.. ta dah
You can use rope to attach them to the cage either in the middle or against the bars, even upright as rats can get a grip on the rope to run up them, choices are endless
It takes about 10 minutes to make one.