a 3d printed flame would work well and is achievable.

You could design it so it is available from Shapeways along with a heap of other pinball parts and mods and they have a material that is excellent for shining light through - detailed plastic. It is opague but put a coloured light behind it and glows up nicely.

here is my tail light mod for the creature which there is a detailed plastic lens in it which has a white led behind the pointy bulb section and a red led behind the flat brake light section.

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here is the site if you are curious of the possibilities.
https://www.shapeways.com/search?q=pinball&sort=newest

with jorro's help we came up with a flasher dome that you could fasten into place....

would be a semi transparent white look but with a coloured LED would look like a rocket flame. It is different from every angle so you could install 1 of 2 ways for a different look or go to the hobby shop and give it a watered down paint job (yellow, orange and red) so light can pass through and look realistic

thoughts......

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lol, I am in Aus.... but really appreciate the offer, I have some semi transparent and printer so printing is not a problem. These would be better printed on a resin printer as then there will be no hex fill which would look heaps better than a regular pla/abs printer.

interesting design challenge though

cheers, hollowed out with a starburst pattern effect - thought it would help light it up better.

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please get a photo before it goes or at least installed in it at the new owners place

wow I was notified that it was sent 10 days ago, maybe it was made in the US branch - strange as Shapeways was in Holland as well as the US, unless everything is now made in the US now ???

total bummer