If you are planning to make an 800hz horn I would recommend the larger 1.750/1" flange. If you are planning on a smaller version similar to the 12" horn, then the 1.500/1" flange is better. If you decide to turn your own. you can use a 1" to 2" conical flange and that will give you a 2db gain. Yes, prices are USD. expensive but the best there are. shipping is included if you can wait for surface shipping. You can easily turn your own or if you know a friend who has a lathe perhaps you can buy him dinner. The dimensions on the site are all you need for correct angle format. The AT-3 is for 2" format and you dont want that.

The dimensions for the wood horns are whatever you want to make them.
The overall dimensions for the horns are on the website along with the flange dimensions. You'll have to reverse engineer it and make any changes to meet your own requirements. I would think the 800hz size to best suit your needs.

Bill