My mat is still in use (film is washing in the darkroom) but I've made a home-grown developing mat to hold ULF films, like the Jobo mats that are now out of production. I ordered some vinyl sheets from TAP plastics and cut one to size (I soup 14x17), with a bit of extra margin. Then I added some small corners and edge stabilizers to make sure the sheet of film stayed centered on the mat. It worked beautifully and I now have ridge-free pyro-developed ULF sheets. I'll post some photos of it later.