There are many different versions of do-it-yourself boards, troughs, or rulers to measure your catch. There are also some commercially available products and all work fine. This is just one I came up with and it has worked out great for me. Because I fish fresh and salt water, I wanted something that would be wide enough and long enough to handle most anything I might catch. This is how I did it.
I started with a scrap piece of 6" pvc pipe. I used a level as a straight edge to mark the lines down the outside of the pipe.
I used a jigsaw to make the cuts and it worked out fine but use whatever you have as long as it is safe. I just eyeballed the width so I don't have a specific measurement. I actually cut it a little too wide at first and ended up cutting about an inch off. Keep in mind the longer and wider it is, the heavier it will be.
Then I used a piece of cutting board to create an end cap. I stood the trough up on end on top of the cutting board and traced the U-shape and then just drew the rest. A little warning - I don't recommend using a jigsaw on the cutting board as it will heat up the plastic and it will melt. I wont tell you how I know that. Ok, once cut, I used marine glue and stainless steel screws to attach the cap to the end.
Then I just applied a stick-on fish ruler from 321Fish.com and cut to length. I went with 36 inches but it obviously can be cut to any length. I guess if I routinely start catching fish larger than 36" I'll consider building a longer one but again, the longer it is, the heavier it is.
In action.
That first fish is about 7 pounds so she fills it up pretty good but the cupped shape really helps to cradle the fish. The second fish was maybe 3 pounds or so just as a reference.
Another word of caution - this trough DOES NOT FLOAT! I recommend attaching something that will keep it afloat or at least attach a leash. I drilled a hole and tied a length of nylon rope or parachute cord and I attach it to a pad eye with a carabiner. Another option is to add a bungee to keep the fish from flopping off while you are getting the shot.