Dear Reshmy,
I am sure , you have found the best format, suitable for your requirement. However, since you are designing, request you to ponder a little.
What would you want to know from, someone who worked atleast 8.5hrs for 5 days week, doing what he or she was hired for, during their employment? We know, most often its the money, what bought them here. Yet, what kept them, within the organisation, till they were there? What were the things that they looked forward to every Monday, inspite of the monday work pressure ?
There can be a zillion reason what makes an employee move out of the organisation. Most of them are generic such as money or growth opportunity. What was the threshold reason leading to the decision to change the job ? You are most likely to come across a unique reason for almost every employee.
If you focus the interview to find the faults in your system and an 'after-action-review' , you are most likely to find the answers supporting your questions. Please do consider as a 'relationship-buiding' initiative, as much as you did when the talent was first short-listed and interviewed. If you plan to build an organisation alumni, then the exit interview, is the best time for you to start. A talent leaving today might be a better fit for you tomorrow or even a connect to the talent pool you would want to hire. Please consider the infinite possibilities that can be there. Wish you all the best !