I am a big fan of PRO Outfitters frames and oar stands. The oar stands are speedrail fittings, and they make the towers in whatever length you like from 8-12 inches: Pro
. These are used by many Grand Canyon companies, and I have used them all over the world on tough whitewater rivers.
You can remove the oar towers easily for transport or stacking boats, and they are a bomber setup. Like lhowemt says, although you can move the speedrail stands around, it is a much better idea to not do that and instead remove the towers when you need to stack boats.
I just got some 8" oar towers from them and wish I had gone a bit longer. I sit on my cooler (no seat, just the cooler) to row.