A cover isn't manditory. If you want a screened one for birds or other critters or a covered one for cold or initial pests, you'd make it large enough to cover your box/es

Some crops are warm season like toms, peppers and eggplants, and others like cooler weather like lettuce, broccoli and peas. Lettuce, for instance, will bolt (go to seed) in hot weather. In terms of spacing, some slower growing summer vegies like tomatoes can have smaller, faster growing things like radishes sharing the same space. The radishes will be ready to pull when the toms are needing the space. I know it's time to put in some cool weather fall crops like broccoli and peas when all of my summer vegies look like they've just hung up the white flag.