You you have to ask youself a few questions:
Which elements in the tcp are stretchable, and which are not?
Stretchable elements are mostly the name label, the meter, the record input label, and maybe the volume/pan/width label.
You should not stretch the buttons, as they are mostly fixed sized.
So what you have to do is, attach buttons either to the left or right edge of the tcp, and attach stretchable elements to the left edge and make them stretchable to the right edge.
Here is a simple example with two buttons and the label:
set tcp.size [200 100]
set tcp.mute [5 5 20 20 0 0 0 0]
set tcp.label [30 5 140 20 0 0 1 0]
set tcp.solo [175 5 20 20 1 0 1 0]
so
[. . . . 0 0 0 0] is left
[. . . . 1 0 1 0] is right and
[. . . . 0 0 1 0] is left, stretchable to the right
edge attachment
[. . . . left top right bottom]
I don't know what your plans are and there are for sure many other ways to design the tcp. So this is just a basic example.
Read the well documented WALTER themers guide for further instructions,
downloadable here:
http://www.houseofwhitetie.com/reape...mers_guide.pdf
Edit: Oh i just see your screenshot. Looks nice! From what i can see, your mcp width knob has a wrong size. Could have many reasons. It looks more like a square.