It is very simple to build and you can tune it in your shack room (simulating a holyday installation in a Hotel...) using an antenna analyzer (I used my Autek VA1) but I think that you can do it also with a SWR meter and a general coverage rtx using low power.
Start with the ends some cm longer (ex. 400 mm instead of 330 mm) and then cut both sides until you have the resonance in 20m band.
Remember that the frequency is not critical because then you will "tune" the dipole with the Antenna Tuner Unit.
Thanks to author Guido, ik2bcp / iu2r / ab9dg