The way you have it set up will work. Try to figure out what skimmer you're going with first if possible.
With the sump I just made, I left an 11" section for my skimmer (but, it's a 40 gallon sump with an ASM G3)
Also, some skimmers work a little better in shallower water (5-9 inches), but that is an easy fix with a small stand under the skimmer. Just make sure there is enough room above the sump to remove the collection cup as well.