Also, I must say that I find the design with a tuned circuit inferior to a broadband transformer as in @HB9BCB 's EFHW (Trap EFHW and Coupler - #6 by VA7JBE and https://hb9sota.ch/wp-content/uploads/2018/01/Slim-EFHW-Koppler.pdf) and the 40-20-10m design described by VK3YY at https://vk3yy.wordpress.com/2014/08/31/end-fed-for-40m20m10m/ (3D-printable parts at Ultralight Endfed Triband Half-Wave Antenna by mfhepp - Thingiverse).
Heinz’ design does not require any adjustments for a decent SWR in a wide range of configurations and over varying types of ground. I really do not see how the new proposal is in any way superior over these existing designs.
If you don’t want to build it yourself, hamshop.cz (@OK1CDJ) has a similar antenna readily made for ca. 25 EUR:
73 de Martin, DK3IT