That, sir, is a very overpriced humidity dome. You can get something like that for a couple of dollars at a local garden store. As for your question, it is preferred to have a tray with slots for each cube because they need to stand upright. In a squeeze, you could make it work without a tray if you propped each clone up with something and made sure it wouldn't fall over.
Look into FloraFlex Incubator domes. They are also pricey compared to a local garden store but they are actually high quality, tough and hold a seal. They also sell their 40/40 cubes to go with the dome. They're great. Thats the combo I use for cloning and I get a 95%+ success rate.