I was modding today and am down to three 510 battery connectors, all three with destroyed rubber gaskets. I started snooping through my collection of odd stuff I should've thrown away long ago, I came across the rubber caps that came with my 510 cartomizers. I don't use them since I like keeping it open for dripping, so I went after it with my tiny Fiskars and a craft knife. I whittled it down to perfect size, popped that center into the connector and it's perfect!!!! Hooray, now I can salvage all these orphan connectors or replace ones that erode over time. 
Liz in Indy
Liz in Indy