It's in the monster manual. Just take the monster then look at creature improvement by type and there you go, the feats are in the back as well.
I can whip up monsters in less than five minutes.
Ah, that's the same sa the "level slider" in 4E. Which does the same thing in ~ 30 seconds.
I guess when people say "build monster", they mean different things, depending on purpose and depth.
Because I enjoy it, my RPG monsters are 90% custom to my own game world. I build them completely from scratch. I did that since 2E. I spent many years doing same in 3E and more recently in 4E.
Maybe I was doing something wrong in 3E (don't know what if so), but I felt that the speed and quality of my monster creations improved dramatically moving to 4E. I do miss the more exploratory "what if I add a half-dragon template, mwahaha, my creation lives!" nature of 3E mosnter build. But the results for me in 4E are better.