I've got the same JMI NGTscope and tried attaching a variety of guidescopes.
Heavier ones, like this stellarview:
http://www.softwaregod.com/jmi/guidescope1.jpg
...mounted with an adjustable 3-point connection to the struts required such heavy counterbalancing it ended up interfering too much with normal use of the scope. What's worked best so far is a relatively lightweight cheap 500mm camera lens hooked to a eyepiece adapter and then either to an eyepiece or an ST-4 with a flip-box and an illuminated eyepiece.
http://www.softwaregod.com/jmi/guidescope2.jpg
A Canon DSLR on the opposite side of the scope, or a heavy televue makes for almost a perfect balance without any extra counterweights. The 500mm is sufficient magnification for ST-4 tracking, but is slow enough to require somewhat brighter guide stars.