I also used demo charges profile for mines, although I feel that they could have expanded a little bit in the usage of explosives in the core book. For example, demo charges appear to have penetration 0 wich seems odd to me.
I think the best answer would be to keep the base Pen 0, but to have it increase by 2 x DoS on the Test to place the charge. Skilled use of demolitions should be rewarded over just tossing the charge at the target.
I'm not sure pen is the way to go with improvised mines - a demo pack is not a shaped charge and so relies on pure destructive explosiveness to deal with targets.
Shaped charges (armour piercing that is) can be seen to have a lower damage in exchange for their armour penetrating ability. Compare the frag and krak missiles for example.
I'd need a book in front of me to concoct a proper, balanced anti-tank mine but i feel it would need to make use of special rules to either automatically damage the motive systems of the vehicle or something similarly cinematic and useful.
Explosives can be combined with other objects to make shaped charges. As is the case in IED EFP (improvised explosive device, explosivley formed penetrators). So perhaps instead of increasing PEN based on degrees of success you could have the person make a more difficult tech use test to assemble the device, given of course they have the proper materials on hand.
You mean something along these lines:
"I shall add this thick copper cone on it's tip over the demo charge. So when it explodes it shall form thin copper spray that increases the penetration of said explosion." or "I shall place this krak grenade inside sack full of ball bearings to make improvides anti-infantry trap."
Know your physics, watch your science channels, go through interwebs or be an combat engineer. Also remember if your players come across UXO (Unexploded ordnance) they might have some fun, if they're crazy enough, fiddling with it to make improvised bomb.
Muutokselle annamme Elämän, Elämälle annamme Muutoksen.