It's a two-step process. First identify the style of fuel cap your Entegra uses, then find a locking one of that type. The style depends on the filler that was used and may vary is diameter and venting (or not). An auto parts store may be able to match what you have and get a similar locking type. Or maybe Entegra customer support could tell you what you need as far as size and vent requirement.
This previous topic may help some:
https://www.irv2.com/forums/f278/any...ap-339238.html
or maybe this one:
https://community.fmca.com/topic/7770-fuel-cap/
Doesn't an Entegra have a locking fuel door?