Need help with a CO2 seal on a vintage pistol.

I recently purchased a HY Hunter American Luger from AoA. Too cool and rare a pistol to pass up.

Long story short, the pistol is 100% functioning which I think is great but is missing the seal for the 8gram cartridge up where the piercing pin is. As such, I've managed to get a few good seals on some CO2 carts with an o-ring or rubber piece. But these are one time solutions that usually don't work. I've emptied more carts than I've been able to shoot. 

There is very little information available on the parts for this pistol and zero availability of parts anyways. 

I've tried the seal off a Crosman 2240 valve, but the rubber is too soft and stretchy in this pistol.

I am open to other materials to try for a seal, but I don't really know where to begin as the issue seems to be most seals aren't strong enough to hold the pressure. Any thoughts for a solution here?


 
Here's a couple of pictures of the layout.

LugerCO2 - 1.1651872326.jpeg


LugerCO2 - 2.1651872351.jpeg

 
Well I found a bit of a workaround that appears to work well. I have some hop-up parts from airsoft and one of the buckings fits snuggly around the neck of the CO2 cart like a collar. It makes a solid seal once screwed in. These are usually in the 70-90 durometer range so should hold well.

I don't hear any leaking and the pistols been holding CO2 solid now for about an hour.