Installing a new 240v receptacle - what do I need to know?
Be sure you use a 30A double-pole breaker to adequately protect your wiring. Make sure you provide an equipment ground and use a NEMA 14-30 receptacle.

The box can be metal or plastic. Some form of cable clamp is always required, it's just that most plastic boxes have an integrated clamp (that finger-trap style door). 

If using NM cable, The cable must be supported and secured by staples, cable ties, straps, hangers, or similar ﬁttings designed and installed so as not to damage the cable, within 300 mm (12 in.) of the outlet box. If the wire is being fished through a finished wall and the wire cannot be secured in such a manner, this rule does not apply.



Pure Opinion

As far as selecting an actual box, from experience, I recommend getting a 2-gang plastic box (or something equivalent in size if you want to use metal). The larger the capacity the better because a NEMA 14-30 receptacle with four #10 wires is not only bulky, but can be a very stiff pain to manipulate. They even make boxes that are "specially" designed for this purpose.

If you use a 2-gang box, you'll want to get a faceplate that is designed to match (faceplates for NEMA 14-30 receptacles are also manufactured for 1-gang boxes).