However you go about this it is unlikely to be a cheap solution. For a mixer you could use a MiniCircuits MDB-44H, $11 each with MOQ of 20, expensive if you only need 1.
Use 20GHz as the local oscillator or the spurious outputs will be terrible; they will not be too good any way. You will need a good filter to reject the 3rd and 4th harmonic of the 8GHz input. If the 20GHz signal contains amplitude inforation then depending on the frequency range you want to cover you may have your work cut out removing the spurs
Dividing and trippling wold work if the signals are just cw, but finding a 20GHz divide by 2 may be OK, I've not looked. A trippler with 30GHz O/P will be more difficult, again I've not looked, but it will be expensive. The device BigBoss found was a doubler much easier to find.
You could make a trippler, but unless you can use chip and wire assembly I doubt the performance will be very good. Beam lead diodes if you can still get them may work.