I'm sure that I'm not the first one to do it this way but I was looking for an alternative way to stretch a frame versus butting the ends of the cut frame rail sections against one another and lapping a strip of styrene over the joint. Here's that alternative:
At the Joint of the frame, remove the shaded portion of the frame rail ends as shown below.
On one frame rail section, remove the center of the vertical portion of the rail leaving the horizontal portion intact (left). The length of the portion removed is not critical, 3/8" or so should work.
On the other frame rail section, remove the horizontal portion of the rail leaving the vertical portion intact (right). The length of the portion removed should be the same as the length of the portion removed from the other frame rail section. Note: Do not remove the horizontal portion by sawing along the inner edge of the horizontal portion of the frame rail, you will remove too much material from the tab. Carefully cut the horizontal portion out with an Exacto knife, etc.
Connect the two sections like puzzle pieces and glue. I use a straight edge (in this case my miter box) and a flat surface to align the sections and keep the rail straight while the glue dries.
Once dry, you should have a strong, straight joint that requires very little filler and is more realistic than the lap joint method.
Hope this helps,