Sorry Wayne, not just for coral. I thought you were referencing a fish QT! Coral do not need to seek refuge.:wildone:
I'd just go with egg crate and pvc pipe as you referenced. I zip tied all my racks into the PVC and it's in 4 quadrants. Probably would have been easier to just make more sections and not zip tie the pvc so it makes for easier removal of sections for cleaning under the racks. To make sure you don't have settling in the frag tank I'd recommend adding a small powerhead under the frag rack so it keeps water circulating and nothing settles out. Even with the strong flow from the two MP40s at full blast and the return under the egg crate does not seem to have near the flow I'd prefer.
I'm really not sure why you'd want sand unless you have a certain coral you want to grow on sand so it does not start attaching to the eggcrate. In that case, you could easily put a small container of sand in it, place on rack, and then you've got a mini-sand bed for a few of your LPS.