55 gallon barrel diameter is a good size for sunfish spawn nest sites. Often the sunfish will create a nest depression about twice the diameter of the length of the fish. Extra diameter space is not a disadvantage.
Nest height of 6" to 10" is probably an ideal height. Experiment with several heights and report your results.
The nest does not have to have a bottom to it, although when placed in a very soft bottom a higher rim would be an advantage. Fill the cylinder with all the same size gravel or use larger gravel/stones in a bottom layer to minimize the top layer from sinking into the soft bottom sediment. the advantage to nests with bottoms is the nest with gravel could be moved if needed.
IMO nonfood grade materials are okay for nest rims if the material is thoroughly washed & rinsed.
