What ToTheSummit said. Mix slowly taking care not to lift the toothpick out of the epoxy to avoid adding air. When it's mixed thoroughly, pop any bubbles you see with a toothpick or a pin. Spread the epoxy on your backing and then pop any bubbles you introduce doing that. You can get rid of more bubbles if you heat the epoxy on the backing with a hot light bulb. Pop if any come up. When you put the cab on top of the backing, start from one end and and slowly lower it at an angle on the backing to squeeze out any more bubbles. When it's flat on the backing, slowly move the cab in a circular motion over the backing with a little pressure (not much!) to squeeze out the last of the little buggers.
If you want to cheat, spray the back of the cab with black spray paint, let dry and don't worry about the bubbles showing.
I wouldn't use superglue on something like this. You'd have to tell your customer not to put it in water. Water and superglue don't go well together.
