I am not sure what type of vehicle you have? But if it is a third gen Ram than you could do something like this. See pictures attached.
I placed a small 38 mm wide (1.5) by 75 mm long (3) by 3 mm thick (1/8) plate that was Velcro to the dash under the rubber mat. The plate than supported the GPS unit when traveling.
2016 34RL CC; 2008 Ram Mega Cab 2500HD, 6.7L, 68RFE 6 speed, 4X4, Smarty S67