Stainless steel is much more expensive on the front end, but, not in the long run. It's a good investment to my way of thinking.
Our first trays were galvanized steel ... over a short period of time, they tend to rust and have to be painted over and over or replaced.
Then we came up with the brilliant idea of aluminum ... because aluminum does not rust ... BUT ... aluminum is a soft metal and can be chewed and consumed by a chinchilla!
That was almost a big problem and a big waste of money ... we did salvage them by having the maker attach a metal rim made of galvanized steel over all the exposed edges.
Next, we went to stainless steel ... it is the easiest to clean, does not rust, lasts the longest and looks the best. (Do not clean with bleach!)
You might want to check the area you live in for a business that makes custom items out of metal and talk to them about making one out of stainless steel. I would ask them to roll the exposed edges as an added precaution and to strengthen the pan. The measurements need to be exact, so you might want to take the cage with you.
Jo Ann