I just have to repeat myself, regardless of what some police officers are saying. The fact remains, that a home-made/professional-made number plate, is not a RTA issued number plate and therefor illegal. If a cop wants to hit you for it, they can and they do.
By all means, make one and you may never have a problem. However, don't think that because some police officers say it's ok, you can get away with it.
They are illegal, simply because anyone can make any number plate. If the original plate affixed to the car is obscured, then the plate you are presenting as the bike rack plate, is your only means for other people to identify you. Here in lies the problem, as the made up rack plate may be (if you had bad intentions) intentionally made incorrect to avoid detection. If your plate is issued from the RTA, then only one of two things can occur, the plate is yours and belongs to the car, or the plate is stolen.
Don't risk it, just get a proper plate, or if you are too tight to buy a RTA bike rack plate, just unscrew your cars plate and affix it to your rack.
My rack is similar to a Grip rack, but I made it myself and it has trailer lights (with reflectors) and number plate lights. As my rack stays on my car full time since I use it near everyday, I simply have my vehicals number plate bolted to the rack.