Oh, you didn't specify who the cage was for... I though it was a
rat cage.
In case it's for mice, you can get a clear rubbermaid (or other brand) container, and burn plenty of holes in it. I say burn, because with an electric burner, the edges are smooth, but if you cut, this will leave sharp edges, if anything, encouraging them to eat their home.
You can also burn out large openings on the sides and top which you can cover with old cage wire or hardware store mesh. I recommend this much more, the ventilation is better.
For mice you can also put together walls and a top out of mesh/wire and pop it into a rubbermaid container or
litter box.
And in these summer months, I see plenty of old cages in good condition at garage sales for very cheap.
i've built many of my own cages, all thanks to 
