I was considering a spring for the next one. I'm looking around locally for some springs. I have a couple of ways I will be testing. The springs that I tried all ready where simply too stiff so I stuck with the rubber band. At first I tried to make it a true double action which has a spring loaded action in both directions. That required more pull length then I wanted.
If I were to replace the rubber band in this configuration I would put it next to the leaf spring. Two light springs flanking the leaf spring would be ideal to keep the forward tension and avoid the twisting or biasing to either side. This would keep everything together on the bottom and allow for creative changes to the top. The springs wouldn’t need to move the whole 8” to be effective either. Just strong enough to eject the blades. Like a catapult on an aircraft carrier. The rubber band doesn't hold the blade forward as it is and that's why there is a forward catch.
I could also go with a spring on top. The hard part would be satisfying my need to keep it simple. Mounting points need to be beefed up etc. and the heaver the spring pull is the more a rail of some sort is necessary.
If I were to go to a guide rod down the center of the tube then I could use a pushing spring against the blade block. It would have a pocket in it for the compressed spring height. A lot like a rifle's recoil spring. But that's back to more parts.
I'm going for reliability, so if a few more parts keeps it relible than I would be for it.