As mentioned, Velcro would be the easiest way. You can make and install your own little pull out tray using drawer slides like that. They also have clamp on keyboard trays, but they might be too big for just that small interface; a clamp on mouse platform that swivels could work. Another thing to consider is a small under desk drawer like this - https://www.amazon.com/HumanCentric-Under-Desk-Storage-Shelf/dp/B08WYWSXKT