I'm not sure what look you are going for... but,
If you are planning on painting it (black maybe?), consider coating the outside with bondo gel. It is not like bondo putty (less brittle, more durable), and not like bondo fiberglass epoxy (all runny). If troweled with a plastic auto body trowel, it would fill the cracks between the beams to make it appear as solid pieces which, if painted, would resemble steel. It would also soak into the wood and add strength. It would probably take a couple thin coats with minor sanding in between to get 100% coverage.
Is that a 50 cal? I'm not positive... hard to tell from the picture. If it is, and you shoot live projectiles (with mild loads or they will go way too far) the recoil will not be too bad. If I am guessing wrong and it is a 1 incher, the recoil will be fairly intense and you will benefit from any added strength that an epoxy covering might give you.
With blanks and wadding (wadding not advised for full sized guns but needed for sub-caliber guns to get a bang rather than a pfffssst) the recoil will not be too bad with either caliber.