I hope you have not stopped listening to suggestions. Look inside the seat tube and the head tube, think of a way to seal off one side and fill from the other with epoxy and microballoons. It will add weight but you won't have to worry about failure. Measure the size of the tube and fgure out the volume to see how much epoxy and microballoons you will need. Mix the microB to the maximum ratio with the least amount of epoxy. The end result is a little heavyer then foam but much stronger.