We have the same Arkel rack trunks but traditional rear racks (on our Bike Fridays), and they fit perfectly. It does look a bit odd on the seatpost-mounted rack but should work okay as long as you don't overload the overhanging part. Don't know that I'd reverse it as that makes it harder to get into and out of, but that's your call. It's a really nice little rack trunk!
Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow