Sorry for the delay, but here you are.
I learned about it from my tech sidemount instructor Chris Reed. I use a pair of Dive Rite thigh pouches on my tanks. They face inwards and down (close to but not against the body) diving and are very easy to access.
The top buckle strap goes around the tank neck, the elastic thigh strap circles the tank, and the bottom cinch takes a 1" strap mounted under the stainless steel hose clamp. This stabilizes the pocket in all directions. To make the bottom strap I use a foot of 1" webbing and fold it over twice at one end, which I then sew shut with a needle and nylon thread. Nothing fancy or involved, just enough stitches to keep it together. I trim a bit off the open end of the strap after I have it fitted, but you need a few inches of excess tail to cinch the strap to the pocket.
I have 3 sets of sidemount tanks and each one is set up for these pockets. I normally keep my backup mask in one and 2 dsmb's and a spool in the other.
I am sure they're not the best solution for everyone, but for Great Lakes wreck diving they are awesome.