Basically I got a belt hole punch and made two holes about 15mm apart at the top of the back system in between the rucksack straps. I then carefully cut a slit to join the holes together. I could then feed the reservoir pipe through this hole and over my shoulder. The actual reservoir sat in the recess between the back system and the rucksack and didn't fall out once the velcro was secured.
As long as you do not knik the round holes when making the slit between the two holes the fabric should not continue to tear across. The fabric has a think TPU coating on the reverse and this will stop any fraying. The hardest thing was making the 2 5mm holes as you really have to feed and gather the fabric over the handles of the belt punch.
This is considered a user modification and voids the warranty on that part of the sack, if anything else goes wrong outside fair wear and tear we'll stand by the product.