Ad: This forum contains affiliate links to products on Amazon and eBay. More information in Terms and rules
Im not a true model maker, but I have had to fabricate lots of things over the years. My suggestion for getting a nice straight edge is perhaps a little different, but it works for me....I would place the part between a sandwich of two wooden staright edges, clamped together to form a straight edge along the perspex sheet that you are wanting to sand. The edges of these two clamped straight edges would prevent you from rounding the corners as you have, which is unavoidable in my experience unless you have a bit of experience, a lot of patience, and a good eye.
Maybe the other guys have a better or simpler method, but that would be my advice...make a straight edge jig so to speak
I did not realize just how curved the items were......he just needs to work out a method to ensure a straight edge, do you not agree?