Hi Jimster,
I've got (almost) the same sight, and tried a few things, including:
- using the tip of a busted arrow to knock the original sight pin out, leaving a small (but not very clean) open ring
- a Spigarelli square sight pin:
http://www.onlinearcheryequipment.co...&productid=514
- a Beiter sight pin (the larger one, Ø12mm) which is what I'm using now:
http://www.merlinarcherycentre.co.uk...ht_Tunnel.html
All of the above are 8/32 size threads to fit the Cartel sight block. The Beiter took a bit of coaxing but eventually fit; it's been my favourite so far, since I prefer to shoot an open ring, but can stick the orange crosshair in if I need it (it's included as an accessory), without having to wind the whole thing in and out of the sight block again.
If you want to shoot with an open ring (i.e. excercising your brain's natural affinity for concentric circles etc.), I would recommend getting the Beiter - the larger Ø12mm one, not the smaller Ø8mm one.
And if you have way too much money to spend, consider one of these:
http://www.francearcherie.com/fiche.php?PID=441 Have fun!