casting around for a 'recessed' base for a ship model I came across these at Hobbycraft. (online too if you don't have a nearby branch)
They are sold as a three-pack described as 'trays' for somewhat under a tenner (I think I paid around £8.50). They are made of wood so quite rigid but light weight. They can also be turned up the other way to make platform bases if required.
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- works either way up
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- there is a plan - this is the middle size. The boat would fit on the smaller one if required