I have a home made one on the Freestyle, wasn't hard at all, need to make sure you use some form of rubber mounting if going against anything hard or it will crack
It will depend what you mean by a windscreen, if it's a full screen you're after then you're going to have to have it made of glass, provide washer, wipers & demisting. However if it's only a deflector & you can still see over it then you should be okay.