In one of the bands I played in, we bought a piece of chipboard and 4 casters and made a simplified version of these for our PA. Worked well but we only had one so there was a lot of lifting gear on and off the tray.
Very handy especially if you have a heavy 2x12 combo like my old musicman.....i have to admit here that even though i love stuff like this i usually end up making it myself..as bossarea said with his old band ..a piece of plywood,some 2x1" a handful of screws and four casters and you could make one if your anyway handy.... my next project is to make one of those guitar cases that takes 3 guitars because i would like to have my backup strat and tele with me on the gigs in one easy case.....
damnation! i had this idea ages ago when i had to move my guitarists AC30. i says "this thing weighs a tonne, why not knock up a dolly out of ply and velcro the amp to it?". next time i saw him he'd made it using his fine carpentry skills, i could have been a millionare!