If you are completely stuck for ideas you could always make a bin cage, Wilkinsons sell a 64 litre one for £10 that measures about 80 x 40 cm (and you can fit a 6.5" wheel in there). You could always stack one on top of another if you want to make the cage even bigger- I have done that.
You could also try looking on second hand websites like Preloved, quite a few of my cages are second hand and are in really good condition. You can also pay for them in cash