Setting focus time with a variable calendar

I was wondering if you had any advice for setting focus time when my calendar can often change. I choose to have an open calendar because my job is most efficient when I’m able to meet with other people to discuss things so I want to remove obstacles when people try to schedule a meeting with me. This means that if I create a focus time in my calendar in the beginning of the week, it could very likely be written over. What do you suggest with these kinds of things? Should I try to reschedule my focus time? Break it up into smaller chunks?