This is an update to my previous post. After some more head scratching, it seems that creating the half right triangle unit by sewing two rectangles together is not doable. I will try to explain...
In the drawing below, the innermost rectangle represents the finished 10" x 16" block. The next rectangle drawn closely to the first represents dimensions of 10-1/2" x 16-1/2", reflecting the inclusion of the 1/4" seam allowance all the way around.
Already you can see that the angle from the finished rectangle to the rectangle with the seam allowances is not a 45-degree angle. The red rectangle represents what the angle would be if you were to take two oversized rectangles, sew, cut, press and trim. Except trimming is impossible because, again, it's not a 45-degree angle that shoots straight into the corner.
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When trimming half square triangles, you can align the 45-degree line of your ruler on the sewn line and trim them because that important guideline is available on the ruler to make it accurate. There is no guideline on our rulers for this one.
It seems that your best bet is to cut rectangles, slice them in half on the diagonal then sew the pairs together on the diagonal. You will be sewing on the bias, but if you handle them carefully, you should be able to control the stretch. If you construct your blocks this way, you may have to mark your 1/4" seam allowances to figure out where your match points are because the fabric pieces will need to be offset for sewing in order to yield a finished rectangle.
You can also paper piece but that might be more difficult since the pieces will be so big.
Hope this helps and is not too confusing.