I suggested the spring from a bicycle wheel quick release skewer some time ago.
I tried it and it worked great.
I 'stretched' the conical spring along its axis, and also 'opened up' the largest loop at the base of the cone.
It turns out that the axial stretch was completely unnecessary, as it made the spring strong enough to propel my Ti-woody right out of the 'stash (vertically!) upon opening, SO DON'T BOTHER DOING THAT.
Opening up the largest coil of the spring served to hold the spring in the bottom of the 'stash.
Only open the bottom coil enough that it is very slightly larger than the hole for your VC.
I used a pen which would just fit into the 'stash to push the spring down into the bottom of the hole.
I tried it with another spring that I hadn't stretched along its length, and that worked perfectly.
It also completely eliminated any rattling while walking.
The only reason I didn't stick with it is that I feel no need to have my VC 'pop-up', and I'm not that bothered by the very minimal rattle without a spring.
Ask at any bike shop and they should be able to give you one of these springs: