Would it help if you had a webcam pointed at the door so you could remotely verify it's closed and locked? Or do you need some the actual feeling of the door being closed to end the fixation?
The right approach is to teach yourself to be OK with the uncertainty of the door being locked or not. This is what behavioral therapy for OCD, specifically ERP, aims to do.
Also, as jcranendonk pointed out in a separate reply to your comment, reinforcing the behavior does more harm than good in the long run. In my case, I've been able to reduce some of the mental strain by following CBT principles in a therapy session and also on my own; I can't comment on ERP personally, but I do know it's shown widespread promise in the literature. My previous therapist incorporated some of its techniques into our sessions as well, but was principally following CBT practice.