Ok, I understand now how the WG has to be attached. I found a good read (in german, but you can let GG translate for you). This article is about the original AMT-1s. They suggest a crossover above 1.5 on the basis of both distortion and dynamics, very nice article in case you have not seen it.
@Runefrost Here is the F360 project in which there is a model of the new style ESS AMT-1 and various attachments with various angles - for inspiration or modification. Also, the minimalistic attachment for a 90x40 horn is there exported as STL. https://drive.google.com/file/d/134DCacFRwFX-Pg6I6K74h1hJd23JuZ0L/view?usp=sharing, https://drive.google.com/file/d/1pq75TDeQaal3G-a5BG-Y2T6Dei19713z/view?usp=sharing