OP, sorry I don't know of anybody selling that top cap separately. Perhaps JB Weld would work?
The Lambo V4 is a nice intro to VV
vaping. I find it works best with attys in the range of 1.8-2.2 Ohms. These lavatubes tend to be rated 2.5A/10W max, but they can rarely deliver that kind of power. I've measured mine pushing a max of 1.75A with a 2.2O atty, at any voltage setting above 4V. I'm a high-power vaper, and the Lambo simply can't give me, the 12-14W I'm looking for.