Hi. I sympathize with your situation. In addition to getting a "universal remote control" as you said, the following methods may also work, for reference only.
1. Search Google images and find the manufacturer of this remote control to negotiate and see if they can give you a new one. You may need to provide the code/serial number of the current machine. This is the easiest and has quality assurance.
2. Use a 3D printing service to print a back cover. However, you need to have the 3D file of this back cover. If your iPhone has a radar function, you can use some APPs (CANVAS, 3D SCANNER APP, METASCAN) to model it yourself, and then send it to some sellers on Lazada to ask them to complete it for you and print a back cover. This may be the solution that looks closest to the original remote control.
3. Use electrical tape, the black kind, which is the cheapest. not that good, and it will works for a short time.
4. Carve one out of bamboo or wood, and be careful not to scratch your hands.