16gt wither killer

Require 1 layer of 3x3 bedrock and a single air block above it. (made a mistake in box unloader, updated at july 31) From version 1.11.1 to 1.15.2, piston will push further than its piston base, causing wither eye height poping out bedrock layer, while it still mostly works, initial explosion will not be covered. Thanks for Scorpio pointing out not using the center of exit portal.