I leave my fish in the landing net for a good few minutes after landing, if it warrents a quick photo then out she comes, quick pic and then back into the water, i will and have stood in the water for anything up to 30 minutes waiting for a barbel to fight her way out of my grip and swim away to sulk for a few hours under the weeds.
I dont subscribe to any retention method apart from what i have described above.......but please please do not think i am having a pop at anyone this is just my method/opinion.
I dont feel that there is a sutible retention method out there yet, some good ideas but what is there to stop a barbel going belly up in a retaining net/tube whilst you are not looking. I feel that by introducing a retention system it would leave room for the angler to become in time complacesent and may be not notice a belly up fish until its too late.
One day someone will come up with the ultimate retention device that will allay my worries/fears and yes Lee's ideas are very good, i will never knock a person for trying and thinking about a solution.
As i have said before this post is not designed to offend or wind anyone up it is simply my opinion.