A movable bench, like the one by Dutch designer Rogier Martens, is a great idea since a good garden bench is, by definition, sturdy, heavy and usually difficult to move.
The movablity factor makes this bench a better item for an enclosed backyard than an open public space. It would be too much of a temptation for ‘barrow-bandits’ to resist. Still, it’s a clever design. Diagrams for construction are on the designer’s website. They’re not too complex, so even if you don’t read Dutch, you should be able to figure them out.