I have been thinking of how to build an ordinary 6-sided die where the probabilities can be surreptitiously altered during a game. The answer, of course, is with non-newtonian fluid.

The dice are hollow, and partially filled with, well, probably not custard, but some variant of silly putty, a viscoelastic
fluid. It is rigid and holds its shape well enough to bounce, but when left for a moment it will flow like liquid.

Using the Custard Dice would work as follows:

Through sleight of hand, drop the dice into your dice cup so the desired roll faces upwards. Hold still or place the cup on a flat surface for maybe ten or twenty seconds, and the putty dribbles to the lowest point inside. Roll the dice, and the probabilities are loaded to favour your chosen roll.

My brother got quite good at hand throwing unloaded dice
to
hit the numbers you wanted when we were children (or was
that
me? some things get a bit vague at this distance) wasn't
guaranteed of course, especially when trying not to look
like
you were trying to do it, but the odds were a little better
than average when paying attention to the dice, a cup
makes
it more difficult of course but with practice you could get
slightly better than average odds with that too.

Less chance of getting caught if there's no physical
evidence, but even card counters get caught so you'd be
spotted eventually.

They used to think darts was a game of chance too, was
even a court case over a gambling license a pub didn't
have.