Hi Alex
I mount my 70-200 f4 and tripod ring onto the dovetail with this handy little accessory from Bintel ($30)...
https://www.bintelshop.com.au/Images/Stock/7734X.jpg
You can find it in the "Mounts" page.
As you know, the 70-200mm isn't the largest of lenses though so something a bit more robust might be needed for your 300mm.
Cheers
Doug
