Simple, Variable High Current Dummy Load
Another approach is to say to Hell with elegance and just brute-force it. Get a bunch of power resistors and hook them up. For instance, DigiKey will sell you 5-ohm, 10-watt resistors for about $1.60 each. Each resistor will draw 1 amp at 5 volts, and 30 of them will cost about $50. Set up an array with some way to connect them independently and you can provide your load in steps of 1 amp, which I'd guess is fine enough resolution for your purposes. It's crude, but probably quicker than messing around with transistors and heatsinks and all that.