There are Terran Protoss and Zerg beacons already available for use.
If you would like to use something else, make a structure with the desired model.
Set the unit to invulnerable, unclickable, unselectable, uncontrollable, and no view on the minimap (cant remember that flag name atm).
Change the model to whatever you like, and there you are. You have a beacon.
Edit: Oh also forgot to add, change the footprint pathing to 0x0 or (none) so that units can walk over it.