Marmox board

[carlos21]

Hi. I have just put a new pvc door into old stone wall, i was hoping to insulate the inside revesls but there isnt enough room as id like, so i was thinking marmox board, my question is how ridgid is the board at small thickness ie 6 or 10mm? 

Because the stone work beneath is uneven iam wondering can i get away with not renderjng it flat behind the board?

Thanks carl