I use pipet tip boxes lid. Nice polypropylene. Cheap and good size for mini gels.
I have a glass container that has been scrubbed acid washed and kept free of as much contaminant as I can for when I need to prepare my gels for silver staining and protein sequencing.
For larger gels any freezer box would do. They actually withstand the acid cleaning and the rest. I have a glass container because it is the best size for my gels, but otherwise, even the tip box lid are great.