Workbenches are the best. Not only for stopping the spawns but with a hammer equipped and in the build menu the workbench shows you the building radius of the workbench. And that radius is also the full range of the spawn blocker visually laid out for you to see.

example you could place two campfires that may not overlap and leave a large enough area to allow a mob spawn. Workbenchs allow you to see the radius.

Best plains farm is going to be a island you can throw down 5-10 workbenches to block the spawns on the whole island. Then throw a portal between your homebase and this.