I'm interested to see how the exhausts turn out in the end Sergio, they are extremely red at the moment.
Like your idea of the canvas cover.
Another solution is to use normal paper tissues (for blowing your nose) coated with diluted PVA, White woodworking glue. Being thinner, they form and sit nicely as if weighted, giving a very realistic appearance. (Ideal for canvas covers on soft-skin vehicles for example)
keep up the good work!