The "Crime Free" tag is a nod to the sketchy 2010s-era internet where "100% Safe/No Virus" was a common (and often lying) promise.
On the four sides of your 3x3 platform (at ground level), place sticky pistons facing inward. Each piston should have a slime block attached to its face. When retracted, the slime blocks should be flush with the walls. When extended, they meet in the center, creating a solid 3x3 slime seal. lovely craft piston trap v01 crime free