Finally found reasonably priced extra drive cages/trays for the Antec P101 (plus a less ridiculous looking motherboard!) : r/DataHoarder
Satechi Aluminum External Hard Disk Enclosure USB 3.0 & eSATA, Case for 9.5mm & 12.5mm 2.5" SATA-I, SATA-II, SATA-III HDD & SSD - Review 2013 - PCMag UK
Antec MX-100 3.5-Inch USB Aluminum Enclosure with 1.5 TB Hard Drive - Tested | eBay
Antec: "Is not possible to install another cage" : r/DataHoarder
Antec MX-100 3.5-Inch USB Aluminum Enclosure with 1.5 TB Hard Drive - Tested | eBay
Antec Easy SATA Review - Overclockers Club
Antec 2.5 USB 3.1 Gen2 External Hard Drive Enclosure, Portable Black Aluminum Hard Disk Case for
Antec MX-100 3.5" Hard Drive Enclosure - HDD/SSD enclosures (Serial ATA II, Black, Silver, Aluminium, Windows 98SE / 2000 / XP / ME / VISTA MAC OS 9.X+ Red Hat Linux) :
Antec "EasySATA", Hard Drive Caddy with eSATA Port, fits 1X5.25" drive bay, built-in eSATA
External hard drive enclosures: Review: Antec Veris MX-1 Actively Cooled Hard Drive Enclosure
Antec MX-1 External 3.5" Enclosure - Bjorn3D.com
Antec MX-100 3.5" Hard Drive Enclosure - HDD/SSD enclosures (Serial ATA II, Black, Silver, Aluminium, Windows 98SE / 2000 / XP / ME / VISTA MAC OS 9.X+ Red Hat Linux) :
MX-100 Antec MX-100 Aluminum Hard Drive Enclosure Storage Enclosure 1 x 3.5" 1/3H
Antec MX-1 Sata Hard Drive Enclosure - Review - YouTube