I wish I would have seen that yesterday.
I bought the Sabra ST4 mount with the intention of fixing 2 of the mounts to the bar permenantly via expoxy. These would be in the perfect position to run LD's vertically. With the other two mounts I would leave them as is on the inside for horizontal mounting SD's near-conincident.
The Sabra is cheaper and comes with 4 mounts, but this looks more sturdy and you could use the mounts for any configuration...