Watch out if you do replace the OEM battery with one of slightly different size, and be positive the upper radiator hose (which can get very hot) doesn't touch it!
Thats one reason for the OEM battery tray. Battery life will thank you for that.
Plastic is better than stainless steel as a battery tray, because it is lighter and does not conduct electricity.
In Europe, there are no problems at all getting suitable non-OEM battery replacements. Maybe because there are so many small cars on the road, many being French, Japanese or Korean with those kind of batteries. Having said that, those higher and narrower batteries are more expensive than common types.