Great looking setup. Ive tried the flexi arms before but the ones I used were probably half as thick andvwere no use for a steady speedlite flash.
One thought is you might consider a diffuser that is the opposite way around to the ones you have there . The idea being that most bugs are a 3d shape, but light falls off inverly over distance, thus a flat or convex light source will have more light hitting the middle of the bug and less at the edges. Whilst is you mack a concave light source you are changing the distances to be a little more even. Ive also seen this done with a second diffuser over tge lens hood and curved with the lend body profie, though tht tends to be on the mpe65mm, which has very small working distances