We all have drip pans but this water heater drain pan is really big..unfortunately can catch multiple different fluids at the same time. Also good for oil catch pan on a low car or a water catch pan under a drain that doesnt allow a high edge pan but still needs some volume.
Plastic - Water Heater Pans - The Home Depot
and kind of related if you need not too expensive sheet metal and all you can easily get is flimsy HD stuff you can try restaurant supply cookie sheet pans. Especially if you need aluminum and would like an edge to keep its shape. 18x26 aluminum for $6. Even after shipping still better than HD. Other cake, loaf and form/molds pans can be used as project boxes or just source of pre-bent aluminum, stainless steel or even silicone.
Sheet Pans
And they make for a cheap fan shroud mount.