Making a 3D sign

I'm looking to make a fairly large sign/logo to go on an outdoor wall, but I'm not sure what to make it from.

I'm fairly handy with mdf - easy to cut and shape - so would sealing and painting it make it suitable for outdoors and exposed to the elements?
Plywood seems like it would be tough to smooth off.

Acrylic was another thought, but I don't know where to buy a large piece of it - approx 500mm x 1000mm.