Since one of the goals is to prevent the shooter from looking at the ground, why not set a dowel rod about 3-4 feet off the ground, parallel to the ground. One side would be supported by the barricade and the other end would be supported by a stake in the ground or some other stand. If the shooter knocks off the stick then a penalty is imposed.
The rod would mimic the trajectory of a baddies bullet. I guess there is an issue with a loose cover garment hitting the stick, but it the best thing I can think of.
I prefer spider rolls. And now you've made me hungry.
I've been watching people wrangle with this problem online since 1996. I have never seen anything really practical suggested. I /loathe/ shooting boxes, as trip hazards and as crutches (things that prevent competitors from having to think about cover). I think the best objective solution is the fault line, on the principle that it can be set generous so that any touch is a penalty. I am ok with the use of a separate line judge for this task -- with a well painted line their job is really easy. If you have to look at the line as a shooter, it's self-penalizing.
I SO'd the IDPA method from '97 to 2000, including at the national level. It's workable because of the warning rule, which actually results in very few penalties being assigned, which is a bad thing -- working cover properly is hard and should present a significant challenge. It didn't, in IDPA, and it was further an easily exploited generosity. I don't know how it's applied now.
I personally prefer called cover, but I prefer doing my scenario shooting as part of a subjective working group or organized training rather than competition.
Ignore Alien Orders
On that theme, there are laser levels that are intended to illuminate on a horizontal plane. ($34.99 example from Sears here : http://www.sears.com/craftsman-laser...6&blockType=G6)
Flip one of those on its side, so the beam plane is vertical, then position it like BaiHu described above. If correctly aligned, it should illuminate a line on the shooters back delineating exactly what is exposed to the threat.
I have one of those lasertrac levels around here somewhere. I'll try to find it and test the concept a little bit.
Dave
Motion detector chime.
Limited range and arc means spectators will not have to stand too far away.
It will take some experimentation (about 5 minutes worth) to determine the exact dimensions of coverage. Once the sector is known, stages can be planned accordingly to prevent false positives. Arc is adjustable via electrical tape over the sensor.
Battery powered.
Inexpensive.
Portable.
Weather-resistant models available.
Every time a chime goes off during a stage, penalize accordingly.
However, I do not know whether the muzzle blast 2-3 feet in front of the gun would activate the sensor...
That's the way I knew it. However, especially at larger matches, you needed a backpack for all the benefit of the doubt you needed to carry onto the range. An SO who is liberal with cover penalties was not welcomed by either the MD or the super squad. This was about the same time that round dumping was being legislated and the FTDR was being neutered, and we were generally exchanging the whole 'spirit of the game' thing for 'it's just a game.'
Ignore Alien Orders
Let's establish some parameters.
Environment: the solution needs to work indoors or out, in hot or cold, dry or wet
Cost: the solution needs to cost as little as possible. For a system that will be reusable for dozens of matches, a cost of about $20 per point of cover. For a system that needs to be bought anew each match, maybe $1 per point.
Durability: If the solution is at the price point for "reusable for dozens of matches," then it needs to be durable enough to last for dozens of matches. That takes into account things like, you know, getting shot.
Safety: The solution cannot create a trip hazard or block a shooter's movement when it comes time to pass through the line of cover to the next shooting position.