Hi Simon,
I have a Nexstar 8SE similar to your Nexstar 8i. I am assuming that you have the same mount as my Nexstar, one fork arm. If you do, when adding any accesories to these scopes you have to be aware of how much extra weight you are adding on the mount. If you add too much extra weight there may be some tracking and balancing issues.
I am considering a Orion ShortTube 80 Guide Scope for my Nexstar.
Weight 1340g.
Tube Length: 40cm
Tube Diameter: 92mm
Have a look.
https://www.bintelshop.com.au/welcome.htm