If it's going to be a bird bath you either need to raise it off the ground or put it out in the open so that the birds don't get taken by predators.
Also I would try to find a way of graduating the base so that there is always the right depth of water for them to bathe in. You could do this with some stone and gravel making it deeper at one end and raising the level up towards the other end.
If you use a fountain you might find that the water disappears quickly - especially if it doesn't rain for a while. But if you like the sound of running water you could fit a hose to the end of the pump and direct the water back into the bath.
I have seen some really good bird baths made from things like upturned dustbin lids. I have a wildlife pool on my allotment that is a bath sunk into the ground and filled with big stones I have found on my plot. I have some lillies and rushes planted in it. At first it looked surreal but it had softened and grown into the plot after only a couple of months. The following spring there were tadpoles.
Good luck with your project!