"Norminn" wrote in message
...
I'm working on a project, using "something" plastic for a float on salt
water. Am contemplating using one of the low, rectangular plastic storage
containers if I can cement it closed to make it water-tight. Don't know
what kind of plastic they are typically made of - polystyrene,
polypropylene?

The materials used for those containers are difficult to adhere anything to.
Some can be heat welded.

I'd try to get a block of foam to be sure it will not crack, fill, leak, or
fall apart. I sold block of expandable polystyrene foam will support 62
pounds for every cubic foot of foam. .