Here's an EL Cheapo laser with with manual leveling that would presumably be able to function while adjusted to follow a slope.
- Harbor Freight Tools - Quality Tools at the Lowest Prices
Otherwise some reflective tape with graduations might be a simple alternative to the receiver and sloped laser.
I don't mean to hijack this thread off original topic yet OP asked and this is where it is leading. This should be a thread of its own since there is probably some interest with varying opinions and method for digging a sloped trench. Anyway...
I went and looked at the HF instruction manual. Now they don't specifically say that you can set the level "out-of-level" but their precautions warn you to "level carefully because just having the bubble off the thickness of the scribed vial line can give a variance of 3" in 100ft". And it does not mention an auto off feature if out-of-level.
So maybe you could set this unit out-of level. If you could you could proceed as I describe above with an optical transit with a couple of twists.
This appears to be a visible laser without a receiver/beeper. Therefore you'd need to see the laser and if the target was attached to the dipper you'd need to place the laser behind you, ie the FEL end of the tractor. You'd still need another rod and tape to calculate the slope and set the laser out-of level but that's easy.
Say you want to dig a 2% trench. That's 1/4" per foot drop. Set the tractor on the trench and dig to the desired depth. Set the level at the FEL end but a bit off to the side so the laser strikes the dipper. Level the laser. Now measure out say 20 feet on the ground along your trench line and set up your height rod. 20ft x 1/4" = 5" vertical drop so wherever the laser, (when level) is hitting the rod, adjust the leveling screws until the laser hits the rod 5" down (or up) from the original level reading. The laser should now be set to the 2% slope. Now set the dipper plumb in the trench. For a standard reading I'd have the bucket either fully curled or fully open so you are always measuring the same depth. Place a target (a card with a horizontal line on it) on the dipper where the laser strikes the dipper. Now as you move down the trench, when you want to check your slope, put the dipper in the "measure depth mode" and read on the card to see if you are high or low.
What remains to be seen is if the laser beam is strong enough to read outside. Some cheap (and even some expensive) lasers lack the brightness to be effective in the sunshine. Maybe one could devise a target shade to help out in this regard.
I don't have any drainage project on my to-do list but the next one that comes up I might just buy one of these cheepo lasers and give it a try. The $50 is certainly reasonable and it would save an awful lot on and off the tractor that's needed with an optical instrument.