First, here's a new link for your trapezohedron (yours didn't work because the L of html
got lopped off):http://propnomicon.blogspot.com/2012/02/shining-trapezohedron.html
As for glowing too "brightly," keep in mind that the photographer probably enhanced the contrast/brightness/etc to make the item appear to glow more brightly after shooting or even with a longer exposure, and also that all gitd pigments will fade fairly fast after they're no longer exposed to light.
If you want to do as you said though, and make the area of glow deeper
inside the shape, you could try using a layer of translucent clay on top. Keep in mind that "translucent" polymer clay has to be pretty thin to be very clear though (i.e., transparent), and the thicker it is the less clear and more "frosty" it will be (some brands of translucent will stay clearer than others, though heating to a higher temp even briefly will make many clearer --the Sculpey brand clays may not work as well for that though**).
You may not be able to put sufficient thickness of translucent clay over the glowing part though to make the glow deep enough inside to suit you without making the covering too opaque to be seen thru well.
If that's the case, you could cover the glow or the glow+translucent with 2-part epoxy resin or even with that kind of glue, or with several layers of polyurethane*** (thicker layers can be achieved too by "dipping" the item into a small container of the finish, then hanging to dry...repeat, repeat, if you want).
You could also try the same thing with (untinted) liquid polymer clay though it must be baked to cure.
Liquid polymer clay also will become more and more opaque the thicker it is, and some brands won't stay as clear as others (like Sculpey's brand Translucent Liquid Sculpey
--that one also can't take the higher heat that could help with clarity like the Kato Polyclay version of liquid polymer clay).
Or do some combination of the above.
Or you could just trying mixing something like "Pearl" mica powder (colorelss) into the gitd clay that you'd use for the whole item. I assume the mica powder would give the clay a slight bit of opaqueing but still keeping a reflectivity that would do something similar?
This page at my site has more on the solid gitd polymer clays (and a little on the gitd liquid polymer clays, which you can buy pre-tinted or do yourself) as well as the regular solid translucent polymer clays:http://glassattic.com/polymer/translucents-glow.htm
There's another thing you might want to consider which actually is a bit of "magic" that does the glow-from-the-inside effect you want but is more involved. This was a technique created by Mike Buesseler; he called these "holographic" beads:http://www.polymerclaycentral.com/mikeb/MoreO&E.html (click on the "gold" pendant bead for enlargement)
That technique doesn't use gitd clay, but does use mica-containing polymer clays. Those are a little hard to expain if you haven't worked with them before, but basically they're made by mixing tiny flat clear flakes of mica into translucent polymer clay or into tinted translucent polymer clays (usually during manufacturing).
When all the flakes are manipulated to lie flat inside the clay, a lot of light gets reflected and those areas look shiny...the areas where the flakes aren't flat will appear the normal color of the clay and darker.
For the holographic bead, you'd first "align" the flakes in a Skinner blend**** sheet (Mike says he used Gold mica clay and Gold + burnt sienna mix, but perhaps you could try GITD and Translucent as your two colors). Then roll that blend sheet up into a log or a solid rectangle (or other shape?) starting with the darker end, or one person said the lighter end. Cut one section from the log, 1-2" long perhaps, then stand it on end. Mike cut vertically down the log to turn it into an octagon, but perhaps you could cut other shapes? (or just sand them down after baking to the shape you want...then must sand/buff or clear finish).
To get the best look from the mica clays and the most visual depth, coat them with a clear gloss finish or sand-and-buff them to a sheen or high gloss (electric buffing is necessary for gloss).
There's more info on using mica clays on this page, as well as all kinds of mica clay tricks:http://glassattic.com/polymer/mica.htm
** I've heard warnings not to bake the Sculpey GITD's longer than 20 min, at 270 F maximum, or the glow can be lost. Most of the Sculpey
brand of polymer clays will also darken more than others at regular baking temps, and that will also happen with their translucents (or with gitd, which is just translucent with gitd pigment in it).
You could try baking at a lower temp for a bit longer, or at least protect the clay to be sure higher temps aren't reaching it by using an enclosed or partly-enclosed baking method (tenting with foil, draping a damp paper towel over, putting in a box, burying in a pile of baking soda, etc:http://glassattic.com/polymer/baking.htm
> Two-Color Sheets