You need 2 things :
- the size of your array to be odd numbers
- your starting coordinate of the algo to be odd as well
So make the Array size 39,29
And event 3 replace the 2 first actions by:
System: set x to floor(random((Array.Width-1)/2))*2+1
System: set y to floor(random((Array.Height-1)/2))*2+1