Before you drill holes, put your rudders on, and be sure that when turning the rudders the tiller arms will clear the footstraps.

The reason most Inter 20's have two sets of footstraps is one for skipper, one for crew. Remember these guys do a lot of distance racing and if the wind is right, they often end up double trapped for a very long reach, in the ocean, where you will stuff the bows into some big waves. If you are only going to race triangles, (or windward/leewards) inshore, you can probably get away with only one set for the crew, but when you are solo it's for the skipper of course.