I recently put a new head unit in my car as well, I got this install kit to go with it

Compatible with Scion FRS 2013-2016 Double DIN Aftermarket Stereo Harness Radio Install Dash Kit https://www.amazon.com/dp/B01AC7F60E/ref=cm_sw_r_cp_api_i_VaScEbQS7R1QE

It came with spacers to make up for the gaps, not sure if you can buy them on their own.