Hi Jamie, I have a couple of pieces of Xcluder 6mm x 25mm stainless mesh you could have for free, sizes ~1000 x 500 and ~600 x 600. Could be made into a 100% mouse-proof cage pretty easily depending on size needed. Just knock together a timber frame, cut the mesh panels to size & staple the mesh to each side. This mesh is extremely strong. You could also try contacting Tim Day (Xcluder fencing/Wildland Consultants Rotorua) or Roger MacGibbon (Natural Logic Ltd) to get hold of some larger off-cuts. I built cages out of this stuff for MSc, installed them on the Maungatautari fence & cut holes in the fence to let the mice in... obviously mouse-proofness was pretty important. We used offcuts for holding rodents and others for experiments at Xcluder too, it'll do the job easily. Give me a yell if you need the mesh. Cheers, Trevor