I've got a couple of those Futaba servo trays. They fit the Futaba S148 servo very nicely, but IMHO, don't serve well as a mount for S3003/S3004 servos (though it is included with systems that have the S3003/04 servos).

I mounted my tray by the ends (it's the 3-servo model), and notice that it flexes somewhat. If/when I use these again, I'll make sure that I use all six mounting screws.

I've also built many servo mounts from 1/8" ply (just like Al suggests). It's not hard to do, and you can put servos just where you want them. I put an additional piece of 1/4" x 1" ply to reinforce where the screws mount the servo, just for good measure.

Just passing along my $.02.

Bob