Special Event - Flexible Session Times?

In my booking calendar, I have created a special event for one day only and set the time range and length of each booking.

This special event is for graduation ceremony coverage and I don't expect to book out the full day, BUT I can only work with one family at a time in 90 minute windows. 

The school has the grads grouped together in half hour blocks, with about 30 grads per time block. I want the families to book me for at least half an hour prior to their time slot, but it doesn't matter to me if I am booked at 9:30am for a 10am grad, or at 10am for a 10:30 grad. 

On the client end, it only gives them time options starting at my start time, and then every 90 minutes after, so the booking times are not flexible. How do I program this to make the times flexible but not overlapping time slots?

Sorry but there is not a way to do that with the special events. Basically you are need a separate value for time blocks to do that which isn't availble with special events.

I think I found a work-around.
I put in the start times in the 30 minute time blocks, and then added an option that they will select to add 60 minutes to the booking. Hopefully that will work.