My favourite solution is L shaped aluminium brackets. They are made to go on corners of kitchen units and stuff. Basically I just drill holes to match the servos on one edge and the same holes on the other then cut the pieces off.
I meant to do a tutorial on it but after cutting 36 servo brackets I havent needed any more doing just yet. As soon as I do I'll video the process. - this was when i realised how easy using alumium is, and its superb strength to weight ratio
Things like this can be really easy to do, its just a case of finding the right material and using a bit of imagination.