How to use linear interpolation or other functions to generate the closed boundary to apply some functions to get certain results

This is my M matrix. In my first column, i want to introduce 3 either in 3rd row or in 4th row with respect to 3 of 2nd column as clear from figure. same i want to repeat with 3rd column. i want in each colum to have starting 3 and then next closing 3. so it will help me to generate boundary. Warm regards for all cooperation and guidance.

M=