Meanwhile ... at the Butte Force Justice Bluff

I've made a custom crosshair set, but at the moment, I'm not happy enough with its in-game appearance to release it.

However, something you can do to immediately improve your crosshair's visibility:

Create \team fortress 2\tf\materials\sprites\crosshairs.vmt

with contents:

"UnlitGeneric"
{
"$basetexture" "sprites/crosshairs"
"$additive" "0"
"$vertexalpha" "1"
"$ignorez" "1"
}

This turns off the additive blending, keeping your crosshair from fading into bright backgrounds.

As for custom crosshairs, there are a couple of problems.

1.) There is one file that contains all of the crosshairs. The minigun crosshair is 64x64, all others are 32x32
2.) They are not unique. The grenade launcher and rpg use the same crosshair, the revolver and fire axe use the same crosshair, etc.
3.) There is no way to scale the crosshair size in game. You probably have cl_scalecrosshair set to 1, but TF2 does not have the cl_crosshairscale command to adjust the size.
4.) The individual weapon scripts reference the crosshair by pixel dimensions, rather than UV. This means you can't double the size of the crosshairs.vtf to increase your crosshair resolution. They will just be clipped. As an example, the spy's sapper uses the top part of the medigun's crosshair for its crosshair.

In summary: that sucks. Because you may have noticed the needle gun is way the hell off where the crosshair shows it, stickies and grenades arc and fall in a noticeable right bias to the crosshair center, and they're really just plain too ♥♥♥♥♥♥♥ small unless you're playing in 800x600 like a choad.