To add drawer fillers, please refer to this link:
To edit drawer fillers,
Select the drawer system to which the fillers are added, and navigate to the Drawer System section in the properties panel.
Select the Construction icon at the top of the panel.
Open the Filler dropdown menu, and open the Left Filler and/or Right Filler drop down menu, based on which side's fillers you wish to edit.
To change width of the filler, specify the desired width into the Filler Width field.
To add a gap between filler and drawer side, specify the desired gap width into the Filler Gap field.
To switch off/on filler panels, select the toggle against Front, Back or Side option, based on which side you wish to switch on/off.