I made one from stuff I had in the garage. There is a knuckle type fitting from an air damper that I put on the bucket bracket, a piece of cad plated round stock, an eye bolt and a short piece of clear 1/2" Sch 40 plastic pipe. On the top view you can see the threaded part of the eye bolt sticking through the loader arm reinforcement plate. The ring end is what grabs the plastic pipe. On the lowere view you can see where the knuckle attaches to the bucket.
I was using the bucket to shave thin layers of topsoil off the land down near the creek and had to get the bucket angle pretty close to perfect. Too much and it wanted to gouge and too little it would skim over without removing any topsoil.
Adjustments to the indicator are made with the tractor parked on my concrete bridge. With the bucket level the pipe is moved so it is flush with the end of the rod.
My previous 'Bota had the little steel level indicator with the green and white decal onthe top of the bucket. It was helpful but not very precise.