It took a little research to figure out what kind of gratings were used on Keokuk, as initially I couldn't make any sense of what Corbett showed in the hatchways and on top of the armored coaming for the smokestack. I was expecting the usual wood gratings. These however appeared to be thin flat slats in one direction and round bars in the other. Then I stumbled across this photo of USS Galena. It appears that the ironclads used iron gratings instead of wood. You can see one laying in front of the open hatchway in the foreground and a second one lashed semi-upright next to the hatchway in the rear. No doubt they were quite heavy, evidenced by the rear grating appearing to bow under its own weight.
Based on the Corbett dimensions, the gratings were made of 1 inch by 8 inch bars connected with 2 inch diameter bars (both presumed to be iron) spaced 12 inches between centers. On Keokuk, Corbett shows the gratings installed with the round bars running fore and aft. Not sure that it makes any difference on the hatchways, since they were square. These gratings of course would have provided much better protection from shot coming down the hatchways compared to wooden gratings.
I made each grating by stacking the correct number of bars (cut oversize), end gluing and clamping them, and drilling the holes for the bars. Shown below is the smokestack grating with the bars installed on the rods. I have the center one in position. I then glued each bar in place with thin CA, added shims, and glued the next bar in place (being careful not to permanently attach the shims in the process!)
Here is one of the completed hatchway gratings with its coaming. The hatchways were 36 inches square.
And the grating and coaming installed on the foredeck. The gratings are removable and I plan to follow the lead of the Galena photo (since I have nothing better) and paint the gratings and the interiors of the hatchway coamings white.
I'm sure there was some kind of weather cover that could be installed over the hatchways, but Corbett doesn't include any in his drawing. If someone comes across photos of Civil War hatch covers, I would love to see them!