The problem with the laser system is it doesn't give you altitude within 50'. If you modified it with a second laser that the helicopter intercepts during the climb, then it is feasible that you could maintain the cube. Once the pilot has practiced enough, the most difficult part of this would be to get the position laser pointing perfectly vertical, and getting the altitude laser intersecting the position laser at the desired altitude (within 50'?). Not impossible, but the altitude adds a degree of difficulty.
Using two scopes is theoretically possible, but would be very difficult. The terminology could become cumbersome. Getting into both scopes initially may be difficult. Also, 50' at 2000' distance is about 1.5 degrees of arc. To give the pilot any useful accuracy, you would need quite a bit of magnification. Then the problem of getting into the scope becomes worse. Also, you can use that to maintain the position, but how do you first set up the tripods with any accuracy?
If the DGPS can't work, and the area is relatively flat, I think the best option would be to install three laser range finders on the helicopter, and process the distances to give you position and altitude errors graphically (i.e. cross hairs on a PDA). Fairly inexpensive range finders can give you meter accuracy up to about 2.5km.
Other ideas are possible, such as determining the distance from ground stations using RF or ultrasonics (not sure if that would work at 2000'), however those would require quite a bit of development, and then calibration would be difficult.
There is a low tech solution that may work out. Build two large square frames with a bright light behind each of them. Do the trigonometry and position the lights and frames such that the only way the pilot can see both lights within the respective frame is when he is in the 50' cube.
Based on some calculations on the back of an envelope, if the pilot can look 30 degrees below the horizontal, and can scan between -45 and +45 degrees of the helicopters nose, you need to place the lights on the ground ~3400' from beneath the helicopter, and set the 15" square frames 50' above ground and 85' in front of the lights. All the pilot has to do is keep the lights inside the 15" squares that are about 3/4 of a mile away (line of sight).
Okay, maybe it won't work out, but hopefully those numbers will demonstrate how difficult the desired accuracy actually is.
DGPS or a tall stick in the ground seem like the best answers.
Matthew.