Interesting but nothing really new here, Wilmslow audio have been doing this for decades, IPL transmission line speakers similarly, there are dozens of sites around with plans of varying size/quality/cost/built options and in the US there are more options from Menuscus/Madisound/Parts Express, Finally you can always home brew with help from relatively cheap measurement equipment, REW and one of the many design programmes like Boxsim, Vituix, Xsim etc.
Also a number of the speaker driver manufacturers publish speaker design for their stuff E.g, Fostex, SB Acoustics, Scanspeak, Seas etc.
And there’s probably another dozen or so I could list But if you are interested, the more popular and successful designers include Jeff Bagby, Paul Kittinger, Paul Carmody, Jim Holtz, Curt Campbell, Rich Craig, Javad Shadzi etc.
Looking into this, the DIY speaker community seem to suggest that their speakers, if built well, can rival commercial offerings at 5-10 times the cost, which is close to what the drivers / crossover parts in most Commercial speakers cost (19-29%). The sweet spot seems to be around the £1000-1500 mark which I’ll equate to a £500-10000 commercial speaker. An example is the Bagby/Shadzi Ceramicos that can be built for less than £1500 but use the same (similar) drivers as the Revel F228be Which retails at £10000.