Finally got a
working version.
Lua Code:
<UI>
<Frame name="DrawFrame" parent="UIParent" frameStrata="LOW">
<Size x="300" y="300"/>
<Anchors>
<Anchor point="CENTER" />
</Anchors>
<Layers>
<Layer level="BACKGROUND">
<Texture parentKey="Background" setAllPoints="true">
<Color r="0" g="0" b="0" a=".70"/>
</Texture>
</Layer>
<Layer level="OVERLAY" textureSubLevel="7">
<Texture parentKey="Fill" file="Interface\TAXIFRAME\UI-Taxi-Line" alpha="1" mixin="LineMixin">
<Color r="1" g="0" b="0"/>
</Texture>
</Layer>
</Layers>
<Scripts>
<OnLoad>
local x, y = self:GetCenter()
self.Fill:SetThickness(64)
self.Fill:SetStartPoint(x,y)
self.Fill:SetEndPoint(x-300,y+300)
self.Fill:Draw()
</OnLoad>
</Scripts>
</Frame>
</UI>