Oh I would definitely crate him for that short of a time. Most boxers do quite well with a crate & some actually will use it as their version of "time out"..If you start at a young age, you shouldn't have any problems. As far as crate size, get a big one!!! Boxers grow fast. If you can get the kind that you actually can split into 2 for when he's small. My hubby hates crating Samson, he really has an issue with it, but we don't generally crate him unless he's in a new location..My baby suffers from separation anxiety & until he gets himself acclimated to his surroundings, he will do major damage. Get a nice soft old blankie, lay it the crate, give him a couple toys & whatever you do, once he's in,,,don't fuss over him..Leave the room, let him get used to it & just build up his time in there.
I didn't realize you were planning on leaving him outside due to work, I should have asked, but what everyone has told you is spot on. We are always extrexmely careful with Samson, esp in the summer due to his age, he doesn't do heat well at all. Those cute short noses we love make it very difficult for them..

Crates allow you to leave & not worry that anything is going to happen while you are gone & believe me, that's something you don't want....