Agreed, the Montana is a rampant blighter. I think NazNomad is thinking along the lines of cracking a hole in paving (we're assuming thats what it is) - the Montana would thank you for it and soon swamp your shed, clematis like to keep their roots cool and a pot would not be ideal.
You will need a very very big pot and very good compost and brace yourself it will grow like mad...I've had one in a pot for years just replace the top inch or so of compost every year and feed it with blood fish and bonemeal... another thing that is mad in open ground but a bit more restrained in a pot is the russian vine keep it on the hungry side and it will be far better behaved not a mile aminute but more a gentle stroll
I have grown it before with success but it was in the ground. The patio is slabs but not sure if they are cemented in till I look.
May be better to remove one slab and plant straight into the ground.