Depending what scale factor is used, the numbers will be different. I'm guessing Steve used 15/8=1.875 but the numbers don't match exactly (using Randy's panel dimensions). Donnie's radius has to be too tight though. I was very careful in laying out my arch and with a 32 3/4" width (like Steve) and a 42" radius, I ended up with 21 1/2" top and bottom. It wouldn't take much to bring that down 1/2" per side. Maybe R=41 1/2"?
I have a hard time believing that the exact shape is highly sensitive and critical. I would assume that baffle material is more important. MDF might be too damped compared to hardwood.
I posted a link earlier and I'll post it again. This BB is 1 3/4" thick, made in the US and can be had for $625/pr delivered. The only drawback is that the pieces are finger jointed (looks mostly). Maple is denser than oak and there is much less fussing with glue-ups.
https://www.foodservicedirect.com/john-boos-oil-finish-hard-rock-maple-industria...I've seen affordable BB in Lab Supply catalogues and the like. If I were to start from scratch I might buy wider hard maple and make my own panels. The only problem I generally find is that with panels this wide I'd have to go find a shop and buy some time on their Timesaver to sand and flatten.
If the driver opening is a bit tight, will a slight chamfer take care of it? I might shoot for 14" if that's the case.
Oh, and based on the UPS Tracking Number I received from LII Audio, I am guessing they have a US warehouse that they are shipping from.