Need correct junction box for wired floodlight cam

The floodlight cam has a 5 inch round base. I need a 5"junction box as im hardwiring the camera with exertior wires and need side enty into a junction box with these exterior wires and then the camera unit goes on top of the junction box. Only problem is junction boxes appear to all be 4" and this is a 5" sized base. Do you have to special order a 5" round junction box somehow or does a 4" round box somehow fit this 5" round base?