Dice with 2, 4, 6, 8, 10, 12, and 20 sides are a useful but incomplete family...
Here are the missing dice to help you build new gaming systems!
A D14 made as a Dual D6/D7. The outer shell has 0, 1 or 2 pips on the vertices. The inner die has 0, 2, 4, 6, 8, 10 and 12 pips (grouped by 2 for a better reading) on its faces.
The maximum score of the dual die is the product of the number of faces of the internal die by the maximum number of dots on one face of the external die.
Add the pips shown on the vertices of the upper face of the outer shell to the pips shown on the upper face of the internal die to get the result of the D14.
This die is almost fair without being a trapezoedron, down to 1% error on 1000 trials.