Set three way-points in a settlement. Same as a bunker.

The settler assigned to guard duty visits the way-points same as a bunker.

That way you don't have to clutter up your settlement with bunkers that the guards don't use anyway.

Like guard posts? 1 settler covers 3 of them

Exactly.

But I don't care about the actual structure. The guards don't use them anyway.

Just way-points for guards or even unemployed settlers to be assigned to.