Hi Lars,
For Venus, use a 50mm blank circle. I crease all mine in free software package called ‘the Gimp’. The phase can be constructed using the ellipse tool, you can get it to produce the right phase using the formula: W= 5*(1-2*phase). So if Venus is 60% illuminated, you would put the phase = 0.6 in the formula and the value you get for W is the width of the ellipse.
You can use a similar thing for Mars but be aware that the terminator changes sides- in a normal mirror inverting telescope with north at the bottom, features pass from right to left (i.e from the following side to the preceding side). Before opposition, the terminator and therefore the phase is on the left (preceding side), and after opposition it is on the right or following side. The phase on Mars is not always at 90 degrees, but the value Q in the BAA handbook (or the value ‘maximum phase’ in WINJUPOS) will tell you where the widest part of the terminator lies.
Jupiter is fairly easy, you need to use an ellipse 60mm wide by 40mm in height (it cannot be a circle as Jupiter is appreciably flattened). Finally the Saturn Section has some excellent templates which can be used to draw on with the rings, Cassini Division and the C-Ring all correctly depicted.
If you have any other questions, please feel free to email me.
Cheers,
-Paul