You get the most field, at a point away from the end of the coil, by putting the most coil towards that end.
That is, a ring winding where all the wire is piled up in the same place, as much as possible (given that the amount of wire is limited by how much field you want to get from it, and how hot you can allow it to get..). You don't want a pancake (the turns span too much radius), nor a solenoid (the turns span too much length). A dense ring or hoop or torus is best.
The best radius will be on par with the distance from sensor to coil. The coil can be made larger than this distance, which will give somewhat better field (it looks like the sensor is closer to the middle of the coil, where the field is strongest), but will cost much more wire, and power, for the same strength. Make it smaller, and less wire is needed, but the field is smaller (spacially), making the field at a distance much weaker.
Volts/amps and number of turns are a free variable. Simply use whatever size of wire (and number of turns) gets you the required power dissipation and field strength.
Tim