DIY cage idea: large box turned vertically (for limited space) and add different levels/platforms. Use some sort of varnish/sealant to protect wood from urine (make sure whatever is used is not going to be harmful to the rats!
DIY: Guinea pig cage that was a TV unit in a former life. A ramp connects the 2 levels and there is a sheltered sleeping area plus lots of storage. The guinea pigs' favourite spot to sit is right in the middle of the ramp.