I put these lines in HudPlayerClass.res for the actual xhair itself (you can put them where you want)
	xHairInner
	{
		"ControlName"	"CExLabel"
		"fieldName"		"xHairInner"
		"xpos"			"c-17"
		"ypos"			"c-15"
		"zpos"			"999"

		"wide"			"35"
		"tall"			"35"

		"visible"		"1"
		"enabled"		"1"
		"labelText"		"9"
		"textAlignment"	"center"

		"font"			"xHair26"
        "fgcolor"       "255 255 255 192"
	}
	xHairOuter
	{
		"ControlName"	"CExLabel"
		"fieldName"		"xHairOuter"
		"xpos"			"c-18"
		"ypos"			"c-14"
		"zpos"			"999"

		"wide"			"36"
		"tall"			"35"

		"visible"		"1"
		"enabled"		"1"
		"labelText"		"g"
		"textAlignment"	"center"

		"font"			"xHair34"
        "fgcolor"       "0 0 0 192"
	}
	xHairBacker
	{
		"ControlName"	"CExLabel"
		"fieldName"		"xHairBacker"
		"xpos"			"c-18"
		"ypos"			"c-18"
		"zpos"			"999"

		"wide"			"36"
		"tall"			"35"

		"visible"		"1"
		"enabled"		"1"
		"labelText"		"n"
		"textAlignment"	"center"

		"font"			"xHair28"
        "fgcolor"       "0 0 0 90"
	}

This is perfectly aligned for 16:10 resolutions. You'll need to change positioning and maybe xhair sizes on other ones for proper centering.

I defined the fonts in ClientScheme.res under the ECon Fonts settnigs like this:
        "xHair28"
        {
            "1"
            {
                "name" "crosshairs"
                "tall" "28"
                "weight" "0"
                "antialias" "1"
            }
        }
replace the number (28 here) with what your use, with multiple entries if you need multiple sizes (like I did with also 26 and 34)

You need Fog's HUD Xhair font installed for this to work. You can find it easily enough on the forum.