Cant recall the number plate light setup, but if its rectangular you should be able to fit something like I got, or just attach the one you suggested in the middle.
Fair point re angle, but ideally, what is best is having it higher and pointed downwards so you can see the edge of the bumper, that way you can judge to the mm how close you are to something (its how the xtrail one is). On the mr2 however it would mean mounting on the spoiler, or beaver panel, which would not look good. Be mindful if you mount it on the bit below the bumper, the bumper edge is actually a fair bit more rearwards, so you will be hitting something before it looks very close, IYKWIM. Either way, they are an excellent reversing aid.
Cheers