Crumb blocks, attic window

    I finally treadled enough crumb blocks, 6 1/2" squares to make a lap quilt. I love crumb blocks, they are like mini paintings, each one different and fun. I already did one with sashing. I decided to use attic windows because it will show off the blocks. I looked for some tutorials, and did not find one that was real clear. I don't want to do the simplified, put a half square triangle in the corner, because I already have enough seams. I can't get a clear idea how to add the second piece without starting to sew at the 1/4" mark which is harder than on the open end.
   I am sewing on the 301 because the treadle does not have reverse. When I get my new machine, the 237, I can treadle the strips on also. Any ideas how to sew attic window strips easier, I would love to know. I have 7 done on the wall. 

On the wall

Crumb block pile waiting for its sashing