I've managed in the past to put this on my classic A1200, and after finding myself in the same predicament as previous posters devised this personal solution:
First, I downloaded the archive from the site using my PC, extracted it using WinRAR then copied to a CD.
The spare miggy 2gb HD that I keep handy was partitioned in two then formatted. After which OS 3.1, CD-ROM drivers and a copy of DOpus were installed on HD1: and was used to boot using the boot manager.
Copied everything over from the CD to HD0: and rebooted. After reinstalling the CD drivers back on HD0: plus all my programs like DPaint, Protext, WHDLoad, etc I used the DH1: partition as a semi-permanent storage for a Quarterback backup of AmigaSYS.
But yes, I agree that there needs to be a more simple way of installing AmigaSYS. My method is far too long a process and not exactly easy for those less learned in the way of the miggy and/or HD installing/partitioning.