add edge loops to tighten up the edges. Note that I extruded the main ribs (the horizontal ones around where the holes are) so that I got 4 side faces. I merged the rear verts back and deleted the extra edges similar to what we did to the front verts.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675
I deleted the faces on the outside of the barrel where the vent is.
Then using cut faces tool (with just the inner faces of the barrel selected) I added edges to match the front and back edges of the vent.
Then deleted the inner faces.
Then using bridge, fill hole, and split polygon I connected the outer hole edges to the inner hole edges.
finally, I added some edge loops to tighten thing up a bit.
NOTE. I only had to do this for the right side. When I had it all tweaked I just selected the inner and outer faces for the vent area and duplicated them and then just rotated them 90 degree for the top, left, and bottom. Then combined everything and merged the verts to close up the surface.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675