I would put sashing and really like the idea of extending the corner of the square out into the sashing as suggested earlier. Having the low contrast blocks around the edge makes sense to me with higher contrast blocks in the center. Thanks for sharing and asking for suggestions. We are all helped when we can see things like this and the resulting comments. I have had times where something seemed good in thought but in practice the contrast was not there. I'm getting better but not where I want to be yet.