Does your transit tilt? I worked out a method the past couple of weeks placing a drain pipe at an accurate 1/4"/ft slope. My trench was straight. I don't know if this would work on a curved trench/ditch.
For ditch work it would work something like this:
Dig a portion of the ditch to the elevation you want/need and set the transit there. Measure off 100 ft on the ground from the transit. From the transit sight level to a story pole at the 100 ft marker and calculate the drop (or rise if going up hill). eg. 1/4"/ft is normally expressed as 2% slope for civil work so 100' x .02 = 2' (or 100' x 1/4" = 2ft) (25" actually but 1" in 100 ft is considered negligible for most drainage work). Now lower (or raise) the target on the story pole that distance (from your level mark). Next tilt the transit until you are sighting the target. Your instrument is now an invisible string line set to the slope you want to make.
As a precaution against jarring the tilt I sight a bench mark when the instrument is set to the calculated tilt. This can be a flagged nail on a road bank, tape on a tree or building...anything. That way if you inadvertently bump the tilt just sight to the bench mark and you're back in business.
When I'm trenching I dig the depth I think I need by eyeball then get off, put the story pole in the trench, sight the target and know if I'm an inch or two high or low. Mount up, correct the trench, if high, then push forward to the next boom length.
A friend to set the pole and shoot the target will make this go a bit faster but mostly they are just sitting there doing nothing while you dig 98% of the time. I did not find it too cumbersome to do myself. But ditch work will be a lot faster than the 4' to 3' trench I was digging in hard clay so a helper here would probably be beneficial.
You don't need to work the full 100 ft and have to walk that distance each time. When ever convenient just move the transit closer and re-do the set up. 30-40 feet worked well for my project but I'd go farther for ditch work especially with a helper. And you don't need to measure a 100 ft in the first place; I used that as an example for the math. Just calculate the drop for the distance you can shoot and set the target accordingly. In fact shorter distances can get you around corners, along an arc, etc.
The ditch example is more straightforward than a trench. With trench work you also need to pull in calculations of instrument height above the trench, existing grade and how far below existing grade your trench will be at the given distance in order to set your target height correctly. But the theory is the same; just a bit more math to set the target. I've done a lot of transit work in my career and I still find it helpful to draw a simple sketch to make sure I'm adding when I should be and not subtracting.
My transit is the least expensive Sears model I could buy as a young carpenter 40 years ago, paid $250 then. Now on ebay you can get equal to much better from $10 to $40. I recently picked up an excellent $600 Leica auto level for $65.
My story pole (rod) is just a 7ft tall board about the size of plaster lath with a cast-off broken tape measure taped upside down to it. The box is at the top, taped the back, tape pointing up, and I can extend another 5ft or so if needed (depends on wind). The tripod legs for the rod are two shorter pieces of lath that I spread to hold the rod fairly plumb and they are pinch clamped to the rod. I thought of screwing them onto the rod but the adjstability of the clamp method on uneven terrain and trenches works better.
Maybe this method will help you or someone else. Good luck and happy ditching.