This is a simple square insert for an ammunition box.

I printed the box and the insert in ABS to allow for higher temperatures during storage.

The print as shown is for 9mm Parabellum ammunition. I used the parametric box from Yanev (Tool box parametric: Tool box parametric by Yanev | Download free STL model | Printables.com (https://www.printables.com/model/39729-tool-box-parametric)) with the following settings:

Bottom: 

• width: 157 mm
• length: 157 mm
• hight: 20 mm
• screwhole: 2.85 mm
• rcorner: 2 mm
• tbox: 2.4 mm
• trib: 3.2 mm

Lid:

• width: 157 mm
• length: 157 mm
• hight: 20 mm
• screwhole: 2.85 mm
• rcorner: 2 mm
• tbox: 2.4 mm
• trib: 3.2 mm

Insert:

• Box_Size: 157 mm
• Insert_Height: 17 mm
• Holes: 12
• Hole_Size: 10.2 mm
• Edge_Gap: 5 mm
• Fit_Tolerance: 0 mm
• Corner_Radius: 9 mm
• Clamp_Ratio: 0.08

The following settings can be used for 303 ammunition:

Bottom: 

• width: 157 mm
• length: 157 mm
• hight: 65 mm
• screwhole: 2.85 mm
• rcorner: 2 mm
• tbox: 2.4 mm
• trib: 3.2 mm

Lid:

• width: 157 mm
• length: 157 mm
• hight: 20 mm
• screwhole: 2.85 mm
• rcorner: 2 mm
• tbox: 2.4 mm
• trib: 3.2 mm

Insert A:

• Box_Size: 157 mm
• Insert_Height: 25 mm
• Holes: 10
• Hole_Size: 11.4 mm
• Edge_Gap: 4.9 mm
• Fit_Tolerance: 0 mm
• Corner_Radius: 9 mm
• Clamp_Ratio: 0.08
• Support_Size: 6

Insert B:

• Box_Size: 157 mm
• Insert_Height: 32 mm
• Holes: 10
• Hole_Size: 9.1 mm
• Edge_Gap: 6.05 mm
• Fit_Tolerance: 0 mm
• Corner_Radius: 9 mm
• Clamp_Ratio: 0.07
• Support_Size: 6

The hole size must be selected such that the ammunition just fits in the hole at the set insert height.

The bottom of the holes for the ammunition is oval and the clamp ratio determines how much the oval will clamp the ammunition.

All other parameters can be adjusted to suit your specific requirements.