Hi Pauly,
IMHO there is a much better alternative - ditch the idea of a big finder and instead equip your mount with encoders, a Nexus II (from Astrodevices) and use Sky Safari to show where your scope is pointing - very accurately. With this on say an iPhone or iPad you can reliably push the scope to anything you choose in Sky Safari and put it in the main scope field of view every time. This will cost about $500-600 depending on your scope and the parts needed. There is also a setting in iOS to set the screen deep red, system-wide, so it won't destroy your dark adaptation, yet remains surprisingly readable.
See example below. Once you have used a push-to scope with a heads-up display like this, I can guarantee you won't be interested in big finders. But if you want to stick to an old-school big finder... two considerations - Where to put it, and how to mount it.
On a dob there are some choices IMHO if you're prepared to step away from the scope eyepiece - a finder doesn't have to be up the top.
The first that always appealed to me is on the side of one of the altitude axis, as this won't affect the balance of the OTA if yours moves freely. If yours is stiff... maybe not relevant to you.
The other place is at the back end, on top of the mirror cell. Might seem daft but many is the time I have resorted to simply getting down under the arse-end of a dob and simply sighting down the OTA because some poor dill cant figure out how to align a hopelessly small finder.
As for how to mount it there are basically two approaches:
1. Rings, as you know, on a short Vixen or Losmandy rail. Large ugly and fragile, and will probably damage the paint job on the scope if its a nice one.
2. If the scope has a mounting "foot" of some sort, then you could use an XY adjustable base. There are a few:
Simple one:
http://www.rothervalleyoptics.co.uk/...ide-mount.html
(Vixen makes something similar)
Or these beasties:
https://www.aliexpress.com/item/S797...2709771f&tpp=1
https://www.aliexpress.com/item/S817....7d847a7te6pJN
ADM and Losmandy made a couple too.