We use a couple of 20 foot containers for club storage they are the standard 8' 6" square. Cheap ones may well have minute holes due to salt water corrosion. I am not sure they are a good idea as workshops unless desparate.
I designed a small workshop some, no, many years ago! There was an unintended consequence however. I wanted good light so lots of windows. I made a place for my magnifying lens on a flexible arm, when out out of use it was upright, but the setting sun went straight into the lens and anything wooden nearby had a brown circular burn, only two boats got caught this way.
If you do your own build, then double glazed windows, just the glass, are quite cheap.
regards
Roy