No Problem! I just started some first-time woodworking over the last year and lets just say I won't be posting the results here (sigh). I've been really impressed with the posters in these forums for their design creativity and at the end of the project, the high level of finish. It gives me hope for the future!
Anyway, I'm afraid I don't have much good advice as to how to ensure projects come out square; especially on such a challenging shape as yours. One of the most difficult problems I've found has been the initial design and measurements. I'm ok with instructions and plans, but its a whole new level of expertise on the design element.
Just thinking about your design though, I don't think the top piece would necessarily be shaped differently than what I've seen in your pictures and your description below. For example, if you simply made a square, the end caps would have to be square; just as they would for a rectangle shape. However, your design seems to include a square bottom, but a polygon top. It seems logical the polygon top would necessarily have different lengths towards the top and low end of the end. That would also result in greater/lesser (depending on whether you measure inside or outside the corner) angles at the top than as compared with the bottom. This would result because the top piece not only has to accommodate the square shape of the rectangle/diamond, but also the change in height as measured from the bottom to top corner or even from a central midpoint.
But then, given my difficulty with design, perhaps I've missed the boat here too.
