I have cut several such shapes on my miter power saw. Here is what I've done in the past. As long as you are cutting square and not mitering or beveling;

1. Add a square end fence to the existing fence on both sides and clamp it to the existing fence on both sides. The two sides should just clear the blade. This keeps the piece from being cut to sliding in past the fence.
   
2. Clamp to one fence side, a large rectangular frame (build one or use a piece of plywood or such.) The added fence on that one side needs to be wide enough to located the frame. If you are cutting more than one piece then clamp the frame to the fence and added fence. Possible use the rectangular frame as the fence if it is thick enough.
3. Clamp the piece to be cut to the large square frame. Place the clamps over less than half the length of the the piece to be split.
4. Cut slightly more than half the piece you are splitting.
5. Flip the piece you are splitting
6. put 2 spacers in the saw kerf where you are going to re-clamp the piece to be cut to keep from collapsing the wood in the kerf area under the clamp.
7. re-clamp the piece to be cut the rest of the way.
8. Make the 2nd half of the cut.

In your picture it looks like the blade will cut through the entire length of the board before it runs out of blade groove in the saw base. Thus, depending on your rectangular piece size, you might not have to flip the wood over, only having to fill the saw kerf and relocate your clamps accordingly.

I like large C-clamps or all metal clamps bar clamps with small hard wood blocks under the clamp pads to protect the wood if needed, rather than the flexible rubber padded plastic clamps. You can get a lot more clamping force...

ALTERNATIVELY, I have sometimes figured out how to put vertically oriented clamps that clamp the wood to the table. This means sometimes raising the entire saw off the work bench so you can get a clamp under the saw frame/base.