Last year Halfords was selling off a whole load of roof boxes and boot boxes from a roof rack maker - Rhino maybe?? The stores that I visited were quoting 75% off, so maybe some bargains are still around as they did not seem to be selling at all quickly - they were there in Feb and still there in September!!
The other thing to consider is galvanised tradesmans toolboxes, usually come with padlock clasps and vary in size upto small coffins. Steel is obviously more durable than plastic, but weighs twice as much, which is an issue on a bike rack with limits of about 50kgs (if I remember right).
We have an alloy box made by Westfalia, about 1.5m x .8m x .8m and empty I can lift with one hand, but its been in the wars with welds becoming brittle over the years and did not last well on about 300kms of dirt roads in Finland. I had to use some exhaust clamps to hold it onto the carrier rack, usually just one bolt and a clip arrangement.