Planter Insert, rounded corners, sturdy

I designed this planter insert to fit in some of our outdoor planters and to allow us to easily remove the plants in the winter and bring them inside. It is designed to be a little thicker and more durable than many other inserts. This is an improved version of the other planter insert on my profile. The base has more filet around the outer edges and the inide bottom edge is also rounded. There are 3 holes in the bottom to allow for drainage.I'm working on a version that is parametric, but am struggling a bit the best way to keep the holes centered on the bottom.