Here’s the short version of what you’re looking for:
MKGlow and similar effects are usually what are referred to as “bloom post process effects”. These work by rendering the scene a second time with only emissive colors (like MK Glow) or copying the original scene output and adjusting the contrast to accentuate bright areas. Then you make a blurred version of that texture, which is either going to be expensive, or kind of crappy looking, but usually a mix of both. Then you add that blurred result in top of the scene.
But that’s the only guaranteed way to get nice, even sized glows on any object.
So the alternatives you’ve been trying, like a front culled fresnel or volume fog shaders, only really work well on nicely rounded shapes … specifically a perfect sphere. Anything else and you’ll get situations where the effect fails. There’s just not a good way to figure out how “far away” you are from the original “edge” with out a blur or sampling the screen multiple times (which is what a blur is).
Now there are other alternatives, some very cheap, but all come with a “gotcha” of some kind.
Option 1: Use a “3d sprite”, usually a single direction & camera oriented plane for the body of something, and maybe one or two cross planes and/or camera facing circular glows to “fill in” when looking at the object edge on. Works surprisingly well, and I guarantee you 90% of the glowing streaks you’ve seen in major games that look like they have some kind of volume were done like this.
Option 2: Use “volumetric lines.” This is basically just an more elegant implementation of option 1. See: https://www.assetstore.unity3d.com/en/#!/content/29160
Option 3: Shells. Sometimes you can get something vaguely soft, but not very consistent with fresnel style glows, but if you layer 3 or more of these on top of each other at different sizes (and maybe different opacities) you can get something that looks acceptable, or even almost as good as a real bloom, especially for smaller glows. This can be expensive with higher poly geometry, but you can sometimes get away with just a cheap additive solid color shader for this. Some people like doing this with multiple passes, but it’s better to do it with a single glow mesh that includes all of the shells if possible, and remember to keep all the edges soft so you can push the corners out via the vertex normal! Using a mesh with rounded edges helps the look too.
Option 4: Fake it with a ton of particles. It’s kind of the naive version of option 1, but still fairly effective. Can easily become more expensive than bloom if you go overboard, and hard to control to get a consistent result.
Last option I can think of would be something like the Doom 3 glow / hand drawn outline effect, but this isn’t something you can easily do with just a shader, especially since Unity doesn’t supply adjacency information to geometry shaders. The short explination is on the CPU (or GPU using a compute shader) calculate the edges of the mesh and generate “fin” geometry. Search for “shells and fins” and you’ll get a ton of examples of this using geometry shaders & adjacency data, which Unity can’t do, but which you could recreate on the CPU.
