Super easy to make quickly on any sewing machine. A square corner is just a square cut out of a flat sheet with the raw edges stitched together. If your mattress bottom is more narrow, a triangle is probably the solution, but surely almost any boat canvas book will have directions. Amazon usually shows you the Table of Contents. The nice thing is you can also custom-make your top sheet so that it is fitted at the bottom for a nice fit. Good to do for home linens & crib linens, too. I make them for shower/layette gifts. Another advantage of DIY is that you can choose your favorite sheets for your boat. BradsDeals.com often has great offers. I got my lovely queen Egyptian cotton sheets through them for c. $23 a set.