Draw one out of paper so you can measure the triangle. You can use your fabric with squares cut in half once, just starch the dickens out of it. Make the fabric as stiff as card stock. Then once your triangles are on the quilt sew a stay stitch around the perimeter until you can your outer border sewn on or it is quilted.
If you are just short of your fabric you still have options. You can sew strips to the bottom of your triangles to enlarge them. Edited to add, this will give the effect of a narrow inner border that your blocks are floating out into. I've seen pics of this technique and it looked good but I can't remember if the quilt was sashed or not. It may look odd with sashing.
Another option is to save your fabric for the binding or a narrow border and use a different fabric for your setting triangles.
It seems to me if you don't have enough fabric for the larger square cut twice, I suspect you will still be short for making the triangles with the bias edge on the outside.
Last edited by feline fanatic; 05-09-2017 at 05:23 AM.