Hi,
The provision and delivery to site of portacabins units would not fall within the scope of CIS.
The process of erecting the portacabins or linking and stacking multiple units together would constitute construction operations. However, as has already been pointed out, they would appear to be part of the same contract from the same supplier and both elements would subsequently be caught under a mixed contract. You can see guidance here:
CISR14030 - The Scheme: construction operations: unusual contracts
Thank you.