Some guys just gotta make stuff more better!

This stock sure looks salvageable to me.

Probably the best fix would be to clean and true-up the cavity in your stock.
It could then be nicely filled with a rectangular 'patch' of black walnut (preferably, from a scrap U.S. stock) that will fit the margins closely.

(The patch should be thicker than the recess. You can 'bevel' or angle the edges for a tight fit).

Glue the patch in place with some 'Acra-Glas'. You can temporarily wrap the bedded patch in place with masking-tape.

Once the epoxy-bedding has hardened, you can file down the 'proud' walnut patch to conform with the stock contour.

I don't think 'steaming' your stock will benefit this wound.
A tight clean patch of a similar walnut is a nice repair in a stock that will be refinished.

An example of fitting a 'patch' and rough filing to contour:

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