Here's what I did, to mount an Aimpoint Red Dot (which required a Weaver-type base) to a Ruger rifler with Ruger's dovetails - a .308.
I sorted through a gun dealers box of Weaver one-piece mount bases, until I found on long enough to span the front/rear Ruger dovetails, and thicker on one end, underneath - I seem to remember it being a #90 or #91 base for a Marlin 39 cantilever barrel mount (It was almost 30 years ago).
I machined the bottom of the alloy rail to the same height difference as the Ruger front/rear dovetails (it was a boltgun) , then D/T'd a single hole in the top/center of each male Ruger dovetail to attach the altered base.
Since the Deerfield's factory dovetails are of the same height, you could probably use a Weaver one-piece base for a Marlin 336, filed or machined flat underneath, and D/T the dovetail roofs, as above.
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