The grade as you have is close to Lionel's trestle grade.
I would recommend moving the switch to the elevated loop back to back with the switch at the top and run your elevation from there. That would leave the left side to the bottom flat.(orange)
I don't like elevating switches on trestles.
For custom trestles, I like pylons from 1" dowel stock with a base and a support. For the temporary layout, perhaps just blocks(piers) may be sturdier. You could also make arches using square stock and 3 sides of a rectangle. Some may need a wider base of of 1/4" ply for stability or Masonite(hardboard). Whichever you think will be the quickest to get done.
The Lionel O gauge trestle sets have 22 pieces and rise to 5.75".