Yeah, I agree with everyone who suggests wide exits + distanced spawn exits. Even some Valve maps don't get it right (e.g. badwater's first spawn is fine, spawn you move to after 3rd capture is easy to camp).
I'd also suggest spawn exits of different heights, I've never seen a multi-level spawn successfully camped.
For reference, though Hydro isn't a map spawn camping happens with, most of their spawn rooms are very well done.
Goldrush and Dustbowl are both good and bad. I was going to say Dustbowl 1st blue spawn was my favourite, but the supply locker is easily targetted with stickies, and red spawns in Dustbowl are easy to camp.
Which brings me to a second note. Make sure BOTH blue AND red cannot be spawn camped. I never got why Valve made some red spawns so easy to camp (e.g. goldrush 1 and 2).