How should one create an ephemeral Diffie-Hellman private key?
It would be nice to just get a bunch of random bytes and make those
  into a big integer than to have to uniformly pick a big integer within
  a specific range.


If you pick a uniformly random value from a bigger set and throw away everything that is not in your range until you find something you will get a uniformly random value in that range.

If however you do something like taking a random big integer and reducing it modulo $q-2$ to put it into your range you will skew the distribution and you will lose some security.