It's not exactly rubber. More like rubbery plastic but they fit holes in the case I had to plug and they seem to stay in place for quite a while. Just dock sprockets because they weren't actually rubber.
π Marinco 6365CRN Locking Plug 250V: Secure and Reliable Power Connection Solution
9 Review
Continental ContiVikingContact 7 195/65 R15 95T winter
21 Review
π 50 Pack of P MAP Maple Pocket Plugs
7 Review
High-Quality White Plastic Flush Type Plugs for Industrial Hardware Applications
9 Review