I am using a thing called ROK BOX STRAPS, which is basically a strap with a clip in the middle, half goes on the box, half on the floor, and you just unclip it when you want to take it out. I think I got these from tonkins audio...
I also have an AU with gas. I have got a custom box, designed to fit over the ledge on the left hand side of an AU boot.
AU2 S-pack Owner: Silhouette Black, XR6 Wheels, Tickford Sunroof, C2R rear bar, Checker Plate Floor Mats, Chrome Door Sills, Blue Backlite Dash, Blue Painted Brake Callipers, Advanced Extractors, Magnaflow High Flow Cat, 2.5" Stainless Steel Mandrel Bent Custom Exhaust, Genie Muffler. Sony Explode 50w x 4 deck, Kenwood 140w speakers (front and rear), Boss 1200 watt Amplifier, 12" Orion Sub in custom box. More To Come!!