Sometimes you can get lucky and find actual drain pans close to your desired size. I use a couple of them in my veg closet but they cost almost as much as a flood table. I really like having flood tables for my DTW I just wish they fit inside the tents better. I have no idea why tent manufacturers are so cheap and won't make a 4x4 tent that actually measures 48 inches inside the poles. I mean I do, they're greedy but really... Its annoying.
You could make a simple table with a raised edge and line it with pond liner or roll on bed liner for a custom sized install.