Ha ha, oh you crazy kids!
OK so here are the results of the light diffusion test for the dorsal "barbecue grille".
My first try was typical of me - overly complex and ineffective. I made up a light tube from a clear plastic straw, inserted two LEDs in the ends, made a back reflector from aluminum foil, then added a paper light diffuser in front. The result was very weak light and it still did not eliminate the hot spots. No photos, as I was just cobbling this junk together while sitting on my bedroom floor in the dark. But I may revisit this idea on a future project, as I think it would work with "big bamboocha" extra bright LEDs.
Okay, I dialed the complexity back and went low-tech for the second round.
Before - cool white LEDs behind the red tinted clear part:
https://flic.kr/p/24uXsiD][/url]K’tinga-25 by N.T. Izumi, on Flickr
The cool white light lends a bluish tint, resulting in a purple overall color.
Middle - LEDs painted Tamiya clear red:
https://flic.kr/p/24uwqK8][/url]K’tinga-20 by N.T. Izumi, on Flickr
The purple color is now gone, but the grille is not lit well and the light needs to be diffused.
Some ordinary folder paper was cut and tacked to the bottom of the light box and the insides of the clear red part:
https://flic.kr/p/2f71pYR][/url]K’tinga-21 by N.T. Izumi, on Flickr
After - now the light is diffused, not perfectly, but a LOT better than what comes in the box:
https://flic.kr/p/24uwvvk][/url]K’tinga-23 by N.T. Izumi, on Flickr
https://flic.kr/p/24uwtgF][/url]K’tinga-22 by N.T. Izumi, on Flickr
So the inner surfaces of the dorsal box are now painted flat white. A variety of materials were tested behind the clear area behing the grille. White paint, wax paper, and parchment paper were tried, but all proved less than satisfactory. The white paint reduced brightness too much, and the papers had noticeable grain. I used the stock kit's clear (untinted) parts for the white paint test, but I used acrylic paint so it can be washed off with Windex or similar. I'll look around for some kind of thick plastic film that is milky white, which I think would work best. The hot spots from the impulse engine LEDs are proving harder to eliminate.
Now excuse me while I listen to Black Sabbath's "Paranoid" while thinking my next move.