Make sure the cage is constructed of strong materials and is constructed well. Don't under underestimate the strength or the intelligence of a Monkey. The cage needs to be big. The bigger the better. The height should be no less than 6 feet.
Monkeys feel much more secure in a high cage.
Hundreds of dollars is needed to build a secure and safe primate cage. Don't use anything less than a 12 gauge wire. Never use chicken wire. A safety entrance must be included to prevent the escape of your monkey. If the enclosure is going to be built outdoors, make sure a large portion of it is in the shade.