When you made a custom model of your WHM, what size did you make it. try a custom model that is 400x400 or 600x600.
Also , in a custom WHM, you cannot have models overlay each other. If your arches are in front of the megatree, you must artifically move the arches so they would be below if doing a custom model.
The data in the current WHM has way finer resolution and handles the collision. We might make the save become multithreaded. basically each thread handles a column.
Do i have your latest xlights_rgbeffects.xml? I would like to try testing with yours.
sean@meighan.net