It's admirable that Phil tried to come up with a system that keeps you from completely removing the lid. Problem is, with a sliding cap, I believe you just can't get there from here, engineering-wise.
The pressure applied by screwing a cap onto an o-ring can't be replicated with a slider....unless the cap were to slip downward, ramp-like, then snap into place, applying a lot of pressure to the teflon fill thingy (engineering jargon).
And if you supply sufficient pressure to seal it in, say a pocket, with significant temperature and pressure changes, then the slider would create a lot of wear on the seal, meaning durability would be an issue.
But I got a sneak peak at the Innokin engineer team's work on Ares V2. This ought to do it.
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