Implicit pore simulation

GROMACS version: 4.5/5/2021
GROMACS modification: No
Here post your question : I am runing Go simulations and want to simulate protein entrance to a pore. The pore would be a cylinder that ends with a planar wall at z = 0. The
transition between the cylindrical and planar wall would be modeled with a circular wall of a given radius. Is there a way to do this using the positional restrains ? combining layer and cylindrical restrains would do the trick ? or would add a cap to one end of the cylinder ?

Thanks for the help.