Those "cheater" eye glasses cases are perfect for 510's, 901's.
Be sure you get the ones that have the plastic lining and they cushion the fully assembled PV great.
Another addition I made is kind of hard to describe but here goes. At Lowe's or any place that carries pre-hung doors, there are these cool little black 3/4 tubes on the hinges to protect them until they are installed. They fit perfectly in the cheater cases but because of they are only 3/4 round with a slit, the 510 manuals button is protected from getting accidentally pushed when it's in the case. They are the perfect diameter for the PV's and you can carry 2 in each case without their clanking against each other.
Another case that works great is the plastic one that soar tarts come in (it's a candy)

It holds 2 of those blacks tubes with 510,s and an extra battery slips in as well.
I like to keep my PV's vertical when I'm carrying them so the juice doesn't run into the atty, so these work great.