This is something I started working on when this thread first started. I have yet to place the car on a grade and mark the centres for the various % increments.
It is - as was mentioned - a line level. The intent is to secure the level down with silicone caulking. The colour of the level itself ccould be changed with model paint to blend in with car OR look like an 'interesting' revenue load.
The unsightly hooks on the top will be cut off. The only problem I forsee is the very small (translated miniscule) movement of the bubble between the various grades which I noticed while initially testing it.
I am thinking of maybe doing a tutorial of the project in the near furture if the interest is there.
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The various grades will be marked on the car along the top sides.
The car isn't too unsightly - and can be run in a normal train consist around the layout.