The problem is if you draw a platform that's not close to the water, what do you do then if you only have 2.5m nets?
You could end up with a metre of net out of the water before you start and 1.5m in the water for 60lb or thereabouts of fish to swim around in.
Bear in mind also the more fish you put in an enclosed area like a keepnet, with little or no flow of water through it, the more the fish will compete for the dissolved oxygen in the water within the net.