Hi
I've never used the Nesco myself, but given the shape of it, I'd assume you'd need to go with a K or J type thermocouple to get the right length to hit the bean mass. The Bluetherm Duo or Phidgets both support a K type (Phidgets also supports J type). They are the the two pricier options, but I don't think the iCelsius BBQ would be long enough (or the right shape) to hit the bean mass.
Naturally, you'd have to mod the roaster to get a thermocouple in there, but I found some good pics.
This one looks like a k type thermocouple.
There's another manufacturer who just came out with a longer looking thermistor type probe, but support for that in Roastmaster is a still on my to do list, so for now it's either the Bluetherm or Phidgets. The Bluetherm is certainly easer, but Phidgets can offer a cheaper solution if you're willing to use a laptop as the network host. For myself, I test with both, but I have an SBC for my Phidgets setup as the host.
Danny