Peters link shows that its not about how much pressure you put in as it happened at 65psi and 25psi its about sealing up the tubes and preventing the resin going into voids, in fact if you put in 1psi and there is a gap, you will fill the tubes. What is needed is an engineer to design a cheap expandable plug but I make sure the plugs are in properly and haven't had a problem.
I Haven't tried it yet but also as Peter says adding something with depth would be very impressive as its magnified when you turn it and it bursts out. I'm also having a play with water slide decals that seem to project the image onto the outside of the blank but I haven't got it quite right yet and having wrecked several am getting close to getting it right.
