Speaker cabinet: Enclosure design for WE 755?

I don't know where you are sourcing the LM 755's from, but assuming it is from a dealer or distributor, I would certainly ask them first.
As an aside, I own the LM 755i speakers that contain these drivers and they are freaking awesome.

open baffle.

http://jelabsarch.blogspot.com/2012/06/open-baffle.html

I don't think there is anything easy about cabinet/enclosures for the WE 555 or clones.

There are copies of the horns out there, non being easy or simple.

I reasd an article about Neslon Pass re creating a pair of the larger horns. It think a regular web search might find it. Again, Neither simple or easy and I repeating myself. But he did use plywood.