I've been building and using them for years and they work great to keep your fish alive.
https://youtu.be/FlArSSCRN4U

Great build. I have a similar build and it works great.

One suggestion... build a second rail for the bottom of the sack that is slightly smaller than the top rail. This way the bottom rail will nest inside the top rail for storage. Fill the bottom rail with sand (or dry cement) so it sinks. This creates an open sack for the fish to move about and stay alive.

Without the sinking bottom rail, the sack will collapse around the fish and they will die (I learned this the hard way). The second rail keeps the fish alive and well... until you chose to turn them into tomorrow nights dinner

I made mine out of 1/2" pex tubing. The pex is flexible enough to make a circle (I laid the pex on the hot driveway for about 10 minutes to make it more pliable). My top rail is 18" to 20" and the bottom rail is about 16". Larger than what you need to put fish in the sack, but large enough for 8 to 12 crappie to stay alive.... yet small enough to easily store in the rear of a kayak.

I might build a smaller one (15" top and 12" bottom) for those days when I don't feel like cleaning a bunch of fish... days that I keep 12"+ only.

I filled my bottom rail with dry cement in part because I had a half-sack of cement lying around. There is an added bonus to cement... if the pex cracks and lets a little water in, the cement will harden and stay put.