Thanks for the responses everyone! This was all very helpful, especially yours Weezy Rider. My cutout measures 19.5 x 12, so I'm assuming it's either a non-standard cutout or the cabinet is not a Horn. Today I'm going to contact Allbrands and ask about their custom Sew Steady acrylic inserts. If that doesn't work, I'll try to find a local plastics business like the one recommended by BLAP.