1) I would say a monstrous creature base would suffice, with a plastic stand attached. Some might even use the larger oval MC base (Trygon base).
2) With true line of sight, and the 50% rule for determining if a MC receives a cover save, this is situationally dependent. You use TLoS to see if the Harpy receives the cover save exactly how you have modelled it. So if you model it way off the ground, chances are you will almost never receive a cover save. Model it close enough to the ground and small enough and you can easily receive a cover save from Gargoyles. Such is the 5th edition world we live in.
Click to feed! =)