So you want an isosceles right triangle in the corner?

'Cause it HAS to be a right angle.

The angle would be that of a HST rather the same one you used for the HRT.

The first block below is an IRT. The other two are right triangles (half rectangle units) - you see the differences in the angle relative to the HRT diagonal.

If you want the first one - the isosceles right triangle - the easiest way would be to just "snowball" the corner.

For the square dimensions, just add .5" for the seam allowances to the side length you want.

Place the square in the corner and sew ON the diagonal

It's going to be a pretty small square so you really wouldn't have to bother marking the line.

Check to make sure the angle is correct, and trim the excess off.

If you want the (regular) right triangles (the 2nd and 3rd shapes), you can use the same method you used for the HRT, but with a smaller rectangle.

At least you wouldn't have to deal with the exposed bias when sewing.