You can modify an existing template and designate a template as the default.
Create a Sheet Metal Part Template
On the ribbon, click Sheet Metal tabSetup panelSheet Metal Defaults.
Optionally, override the Material, Thickness, or Unfolding Rule.
If needed, change the sizes of the origin work planes to accommodate the average sheet metal size.
Click File iProperties to open the iProperties dialog box. Set any iProperties that you want files created from the template to inherit.
Save the file in the Autodesk\Inventor [version]\Templates folder, or a subfolder of Templates. The newly created template is available in the New dialog box.
Specify a Default Sheet Metal Part Template
In the Templates folder, move or rename the existing sheet metal.ipt file.
Save your sheet metal file with the name sheet metal.ipt into the Templates folder.