UniFoxOs,
why don't you use a PAPI of the poors ?
Just erect two or three crosses set so that they are perfectly aligned on an horizontal line at the level of your eyes when you sit in your excavator.
That system is used in many small aerodromes (often in montainous region) ; three crosses aligned on the correct glide slope.
Luc
Use a transparent plastic tube filled with water to get an horizontal reference over some distance.
Edit: seemingly, the english term for that is "boning rods"
Paving Expert - AJ McCormack and Son - Setting Out - Boning Rods