speedcubing mats can be quite complicated, many people want specific designs for the aesthetics but many designs are simply not compatible with your timer due to different manufacturers not making them cross-compatible. Usually, either the timer or mat will come with some clips that allow the timer to be attached to the mat so it should not come off by mistake. There are many nice designs that people will like to try. Many of these excellent options are available at speedcubing.org!