A standard Skywatcher 9x50 finder scope is often a popular and successful choice for a guide scope, used with a suitable camera. But the standard finderscope bracket is a bit flimsy and the scope to T-mount adaptors are a bit pricey!
As an alternative, here are two STL files. One is for a robust mounting bracket with a standard Vixen/Skywatcher dovetail to suite any scope with a barrel about 50mm or so in diameter. Use it with six Nylon M6 screws.
The other is a T-mount adaptor that fits the internal thread left when the eyepiece fo a Skywatcher 9x50 finderscope is unscrewed. It does not suit Celestron finderscopes which apparently have a 1mm pitch thread.
It should allow any t-mount fit astro camera such as a ZWO to be brought to focus by adjusting the front lens.
Print the parts large end down. I suggest 1.2mm wall thickness and 25% fill. the threaded adaptor will benefit from 0.06mm layers if your printer can do them.
Take care when first threading the adaptor, it may be a little stiff at first, especially if your printer over-extrudes, and take care not to cross-thread it.
Use these links to download the STL files: