Our unique Boompole Apron attaches to the side of your cart and provides storage sleeves for up to four boompoles, antennas, or small stands. Because it is made of rugged cordura fabric, it weighs almost nothing and folds flat.
Currently, I just use a cheapie bungie cable to strap the boom to my cart, but this is only practical when it's stationary. If you need to wheel around to get closer to the next setup, or move out of the way of lighting equipment (which inevitably happens), it gets a little wonky. An apron like this seems like the most practical way to store booms, short of a far more expensive custom-built cart.
Link to Equipment Emporium.
(P.S. The post at Pro Video Coalition lists today's date, but a Google search brought up a post from Videomaker.com from 1999 that lists the Boom Apron. Seems as though it's a older product with a new announcement making the rounds.)