I seen some like this at a bee conference one year and liked the idea, still tried some other things with them.
So here is what I built from memory.
My hives are 19 7/8" long, so I cut the side rails that size.
Since I wanted some extra glue up area and some thing to help keep the feeders stable I cut the end bars and centers to 15 1/4.
With all the dados on the ends and centers 3/8" deep it gives me the over all width of 16 1/4.
Of course 3/4" wide dados allow a perfect fit of the end bars in them.
You can have the center bars as far apart as you wish and as close as 3/8" but I feel that is to close. I chose a inch. Find the center of the box measure a 1/2 inch on both sides of center for that inch.
Once you have that inch measurement you can set the fence on the table saw to cut the dados. Do one side rotate ends and do the other. Then you reset the fence to do the centers the very same way, one side then the other. I measured so I don't have to do the figureing out every time any longer.
I used construction glue (locktite grab it) to glue the bottom on as well as staple it with 7/8" crown staples.
When that is done you can measure the area for the float. Just make sure you leave some clearance. I left about 3/16" on the sides and ends.
I made the floats out of 1/2x3/4 inch rails and the cross ties are 1/8"x 3/4" , didn't soak these but will all the rest as it tends to stop splitting. I replaced several that did split.
Al