A No KO Custom Stage is one confined by objects which cannot be passed through, where it is generally impossible to be KO'd or SD. These types of stages provide a different twist on the normal setup, as players fight continuously in an inescapable area, without re-spawning and with high damage percentages. They would generally only be used for matches with various externally generated goals, such as being the first to reach a certain amount of damage, or having the least damage at the end of a time match, and are also conducive to the more traditional Stamina mode. A No KO stage might also be used simply for sparring without the need for recovery.
Design Requisites (SSBB)
A standard/typical No KO is composed of a square/rectangular area of basic blocks, with spikes along the top to prevent characters from spawning there. Variations include a triangular shape to cause a slant for items, hindering item usage as soon as they spawn, a more rounded stage to provide an aiming challenge, made of icy blocks, and a roof of drop blocks.
Box is a common type of a No KO stage. It is impossible to get knocked out of a box. There are two main ways to build a box. The first is to simply build a stage-builder stage and make the walls and bottoms of the stage sealed by blocks. Then seal the "top" of the box with spikes and/or falling blocks. The other method is to have the top, left and right sides sealed by blocks and the bottom as drop through ledges, allowing fighters to enter from the bottom and not be able to leave accidentally. In order to make this easier, another strip underneath can be used to land on before jumping in.
Spikes at the Top
Adding Spikes to the top of a box custom stage and blocking off the sides makes it so characters spawn in the center without delay. This makes it so players can't stall at the top of the stage and is probably the best way to build these stages.
Design Requisites (Super Smash Bros. for Wii U)
Building a No KO stage in SSBWU is possible, just harder. The key is to draw the sides of the stage one at a time in a careful way that doesn't allow players to escape, since players can't draw a shape and create a hole in it. Then, by adding a layer of thin lava on the top side, make sure the lava is completely covering the roof.
As the stage requires at least one space between the stage border and the blast line, there are several Final Smashes that allow players to cross this barrier and be KO'd. Ike's Great Aether sends him and his victim to the peak of the stage, causing a Star KO outside of the main play area. Lucario's Aura Storm also sends him to the peak, allowing him to escape, and Pikachu's Volt Tackle allows passage through solid objects and may culminate outside of the intended arena. Snake's Grenade Launcher and Olimar's End of Day also puts them to the stage peak after the Final Smash is over as they come back down from the top of the screen. Furthermore, all 3 Starfox characters' Landmaster put them at the top as well. It is also possible to escape using a Warp Star and if all three Dragoon pieces are collected. Using Ice Climber's final smash near the wall can lead to sliding out of the course.