Look steel tube scopes like this GSO will always have some flex, even the dob models show collimation shift when the altitude is changed, so much so that collimation is recommended at 45 degrees so that there is minimal shift at higher altitudes. Basically, aside from springs to stop primary movement, and the use of a top mounted vixen bar to lock the rings in place, there is not much more you can do with that tube.
However, you can buy carbon fibre tubes that are direct replacements for the GSO steel tube. These carbon fibre tubes come from Germany, and are available from Teleskop- Express. Check their website. The problem of course is that the basic carbon fibre tube costs more the GSO scope did originally. Your present tube components transfer directly onto the carbon tube (focuser, secondary, primary cell and mirror, etc.
Link here:
https://www.teleskop-express.de/shop...elescopes.html
The tube will cost you 538 Euros, or $821 AUD, then add shipping on top of that. (Note: that when Australians buy from Teleskop-Express you will not have to pay the Euro zone VAT tax, which is why the price is reduced to 538). You will get great collimation holding power and tube stiffness for that price. I doubt you would be able to sell of the old GSO tube for much, as a scope without mirrors etc is just scrap steel.
I should add that my 10" f5 imaging newt uses the same mirrors, as your scope, which I bought directly from Bintel (the mirrors), but I built my own carbon fibre strut tube to hold everything. If you can work with carbon fibre and epoxy it might be an option for you.
You can find my build thread here:
http://www.iceinspace.com.au/forum/s...t=imaging+newt