I'm using a Sullivan 4oz. flex tank. It is something that I purchased years ago and never used. It is a tall thin tank with the stopper located straight out the end. I have it laying on it's side next to the throttle servo. The only padding is under the hatch. So far no fuel foaming and the little F.P. is running great. I'm using a Master Airscrew 8 x 4 cut down to 7 inches and the motor is turning just under 15,000 rpms. It won't climb straight up but will maintain an 80deg. climb til it is out of sight. By the time I put 4ozs. of fuel thru this plane I am ready to land. Hope this helps.
Chris