For once, I'm planning to dress to match an event theme, specifically the Roman theme for this year's Red Tower.
My first attempt was hacked together out of some spare cotton, and was basically a truncated triangle. It was basically functional, but not really ideal. I'd made it by cutting a rectangle diagonally and sewing the resulting halves into a larger triangle, which was good as far as conserving fabric, but bad in that it put a seam right up the middle of the resulting garment.
The second version was a more faithful implementation of the aforementioned pattern, using the leftover linen from cutting out 4 pairs of brache. From a 16" wide strip I cut a 40" long piece for the center and cut the remainder into two triangles for the ties. An initial test assembly was functional, but exhibited some unattractive gapping along the bottom edge of the triangular pieces. Some experimentation led me to conclude that the wide end of the triangles should be curved, and I marked the point where the bottom edge of the triangle should meet the center. The resulting isosceles triangle can be seen here.
After this adjustment, the finished subligaculum turned out to be surprisingly comfortable. For the sake of my friends with weak constitutions, I will refrain from posting any pictures of it in use, so you'll just have to take my word for it. And now, I am going to bed.