I can't claim credit for this technique. I recently played ctf_colorblind_beta which used this technique liberally and decided to figure out what was going on in there. Turns out it's really simple. Step 1: Create a cube! We'll use a 64x64x64 to keep things small and easy to work with. Decorate your cube in a solid color. This is important. If your cube has any sort of texture on it, it'll lose that cartoony effect. I used models/debug/debugwhite. Step 2: Create a slab on each side of the cube. These should be extruded from each face of the cube by some set amount. I used an offset of 8, so each slab is 8 units from the matching edge of the cube, and is 8 units larger in each direction, so they're 80x80x8. These should all align so they completely enclose the cube. Texture these with another solid color. I chose models/props_trainyard/black. Step 3: Texture the outer faces of the slabs with tools/toolsinvisible. This will force only the faces facing the cube to be visible. The other faces will never be rendered, and any face not facing you will never be rendered. The upshot is that the cube will always have a black border, creating the shading effect. That's it! This is pretty simple to do for any cube or rectangular shape. I'm very new to TF2 mapping myself, so there might be an easier way to do this using the hollow tool or something similar. Have fun! The attached VMF has a few cubes you can play around with. It's much cooler when you see it in motion than in plain old screenshots.