20 minute appointments with 15 minute practice background schedule

I administrate ECW in a multi speciality practice.
Baseline schedule is 15 minutes- originally set up for primary care. Now our specialists want 20 minute appointments. Since the background is 15 minutes, the creation of the 20 minute appointment leaves ‘holes’- e.g. An appointment is made at 9 AM for 20 minutes…next open appointment would be 9:30 and not 9:20. Has anyone instituted this successfully without the staff manipulating the starting time for every appointment and without changing the baseline 15 minutes slots?
I also do not want to separate the speciality practices into separate ECW practices - they are designated by facility now.
Thanks.