random interchange and capping

hi guys

im trying to set up a new file and having a few issues that i hope someone can help. im fairly new to grasshopper so sorry if its a silly question.

so im wanting to make surface that is then subdivided in a grid. i have a regular grid but would like to be ale to add in more so for example a extrusion that will take up 2,3,4,5 ect squares at random. i then want to extrude the surface to make different boxes. but im not able to cap the extrusion i have at this point not sure why.

the image below shows the grid on a surface and an idea of the references for 1,2,3 or 4 squares allowing the space to be maximum density. 

please see attached files  

any help would be great 

thanks