Bula all,
I am in agreement with Simon and Doug.
I mostly fish alone and if I want to weigh a fish I slide it into a Kevin Nash Carp sling, which can take fish up to 45 kg, and then lift the fish out and weigh it on 100kg Salter dial scales. I don't usually photograph fish, so it goes back the same way.
I must admit I haven't done this with Gts over 30kg, but with two anglers it should work fine. I have weighed a 50kg Giant Clam like this too!
When I used to work on Lake Nasser, we welded a ring on to the canopy of the boat and had the scales semi permanently mounted, so the sling and fish could go straight on. This was great for big Nile Perch. The issue back then was finding a sling big enough, although there are several now available.
On the subject of fish care, English anglers lead the way with unhooking mats, antiseptics etc. Some of this is a little excessive, but I have soft rubber matting in the bottom of my boats, which is nice to stand on and goes a long way in preventing damage if fish thrash around.