You need to create a set-in triangle. Normally, the blocks are sewn in rows and the triangle is added to the end. Then you can sew the rows together and the triangles will be captured. Right now, you would need to sew the triangles in place with a Y-seam. That is not too hard, just tedious.
As for the triangles, you could use a solid (many on-point patterns use a solid) but you can also sew more of your DP9 blocks and cut them to the correct triangle size.