The Collision plate from Relozu can be found in the last entry for "Assets", the one with the geometric shapes icon.
Spanky's mod can be found here: Invisible Collision Tool. He had an older mod but it's no longer recommended for use as the features there are added in the newer one.
I typically use the asset to "lock down" hand-made city quarters (usually buildings with no collision box enabled such as many of the asset mods available here and on Steam) or around transport hubs like train stations/stops and bus/truck stations, as I usually don't want houses or stores to be built near these areas.
Of course the Build with Collision mod by VacuumTube is also enabled to allow manual placement of collision-enabled assets and other entities.