A possible workaround is to use the #24 Depth Render setting.

This will output a B/W image based on how close each part of the model is, with Black being close, and white being background.

Render your model, then render with the Depth Render.

Open both of these as layers in GIMP.

If you set your GIMP color (select all similar colors in pic, rather than the select area tool) selection threshold to 2 and select the white background in the Depth Render, it should give you a selection that neatly trims all the background.

Hide the Depth Render layer and select the other layer. Add an alpha channel (if one isn't present) and CTRL-X to cut the unwanted pixels.

You might want to try to render with the sky as a background color, black. That gives me some decent results...

Twenty second gif walkthrough, click to play:

End Result of that walkthrough:

Just for informational purposes, in case someone hasn't run into the term 'becquerel' before.

It's a measure of activity, in which 1 decay event occurs every second.

Becquerel - Wikipedia

(en.wikipedia.org)

From that link:

@unknownuser said:

The nuclear explosion in Hiroshima (14 kt or 59 TJ) is estimated to have produced 8×10^24 Bq (8 YBq, 8 yottabecquerel).

For comparison purposes (as dangerous as that is) a terabecquerel is 1x10^12 Bq. So 130,000 x 10^12 bequerels, or 130 x10^15 bequerels, or 130 petabequerels, or .13 exabequerels (which is the prefix for "x10^18") of iodine 131 have been released.

About one - sixty one millionth the total decays per second. But that's really apples and oranges, because comparing the relatively slow decay of iodine 131 to an engineered millisecond-length nuclear chain reaction isn't really informative.

Another comparison is that about 3.5 million curies of iodine-131 have been released.

@unknownuser said:

A radiotherapy machine may have roughly 1000 Ci of a radioisotope such as caesium-137 or cobalt-60. This quantity of nuclear material can produce serious health effects with only a few minutes of exposure.

Curie - Wikipedia

(en.wikipedia.org)

So there's the rough approximation of 3,500 radiotherapy sources floating around... dispersed over who knows how many cubic kilometers by now.

Iodine 131 does have a half life of 8.02 days. In four months (120 days) about 3 thousandths of a percent will still exist. In a year (360 days)2.84 x 10^-12 percent will still be undecayed.

Something like this?

Took about ten minutes, and I did it as a challenge for my, still fledgling, modeling skills.

Was guessing at dimensions of the internals, and the walls of the box are only 1mm thick, and the tail is 5mm thick.

Sketchup 8 file