My wife has finished a stashbag specifically designed for the trunk of the ID 4. The bag will fit in either the left or right side and includes a clip on so it will stay secure in the rear for the vehicle. The bag also includes a handy bottle opener.

Here is a quick video of the Stashbag in action.

She currently has them in a variety of colors. If you are interested please check out her Etsy Site.
https://www.etsy.com/shop/SewFullOfFabric