The best thing to do with something as simple as a target box is to simply use EQ to view it. It means having to reload the ui every time you make a mod, but its what most of us do. I have SdlWdl, but I never use it. I use Notepad to edit, its free works just fine and is the easiest text editor to use for simple scripts since it wont try to help you. I use Gimp to edit images.
I suggest starting with the code for the default target box, just expand its over all size to something really big, like 500x500 and start adding pieces to it until it breaks, fix whatever is broken then keep at it. Once you have the layout you like,resize it properly and put the image for the skin file into the folder and then fool with it until its right. There really is no reason to write all the code out for the box when its already written out in the default file, you will just be writting the same exact code with different numbers for where things are located.
Somethings like text tags can be just invented eg.
<LabelItem="Target_LabelX">
<ScreenID>LabelX</ScreenID>
<RelativePosition>True</RelativePosition>
<Font>3</Font>
<Location>
<X>1</X>
<Y>1</Y>
</Location>
<Size>
<CX>300</CX>
<CY>12</CY>
</Size>
<Text>This MoB will soon send you to your bind point</Text>
<TextColor>
<R>255</R>
<G>255</G>
<B>255</B>
</TextColor>
<NoWrap>true</NoWrap>
<AlignCenter>False</AlignCenter>
</Label>
.
.
.
<pieces>Target_LabelX</Pieces>
Will add text to the upper left corner saying "this MoB will soon send you to your bind point" in white #3 font. Feel free to copy this and play with stuff like nowrap and size to see how the game will affect it.
Other stuff like gauges or stat numbers, tribute etc. must have the proper EQtype assigned or the will not work right or not work at all.
GL, remember its mostly patience that makes a good UI. Target windows are probably the easiest things to modify since they are so basic to begin with.