I've got an old Trakker weigh sling with two aluminium rods, which zips at either end (not the new mesh material). This has been great for the obvious but is also easy to put in the river and scoop in plenty of water to allow the fish to rest prior to weighing/photographing. It's also handy for returning fish to the water because you can help the fish stabilise (if necessary, using the rods).
Now, I mention this because it relates to the above posts in a surprising way. There have been several times when I've lowered the sling into the water, opened the end zip, given the fish plenty of time to recover, and possibly because it feels slightly enclosed, it has been happy to stay there for up to half-an-hour when I've returned to my rods! On one such occasion, I'd already hooked another fish and had to shoo the fish away so that I could use the sling! This was of course in well-aerated water and not in a warm margin in heat-wave conditions. The point being, this sort of sling is useful to carry for retention etc. as the fish I've come across seem very comfortable when in them for short periods.