This mod adds the C4 to the smithy, makes it more expensive, and allows you to change the damage C4 does through ini file.

Add the following to GameUserSettings.ini:

[CustomizableC4]
ExplosionDamagePercentage=100

replace 100 with whatever number you want.

100 = 100% of normal C4 damage
50 = 50% of normal C4 damage

Examples

Normal C4 damage to metal is 3516

ExplosionDamagePercentage=200 - damage to metal is 7032

ExplosionDamagePercentage=50 - damage to metal is 1758

This mod changes the value for C4, meaning it works for every structure in the game that can be damaged by C4

If you want to triple the amount of C4 it takes to break structures:

ExplosionDamagePercentage=33