The Jet Black number plate holder slips easily onto Jet Black branded car racks. It is quite possibly compatible with other brands of car racks with some minor modification. A number plate can be bolted easily to it. The number plate can be held on with the supplied bungy cord but it's a better idea to bolt it on. Bolts not included. Here is how to order an extra number plate in NSW
This is the version with lights and indicators and a cable that plugs into your car's outlet for electrical items. If your car has a tow ball then it probably has the electrical socket output near the tow ball for light and indicator outputs.
If your car does not have a tow ball and you're purchasing a bicycle car rack that doesn't require a towball, you may be better off buying the simple number plate holder without lights.