Wow, well $10 was a steal and a half! That would drive my friends at the FB PK Grills group crazy! Yours even still had the bottom vents present!
I was actually working on mine yesterday. Here are a couple thoughts:
This grill is supposed to have a charcoal grate that leaves a space for airflow. It is also important because putting charcoal right on the cast aluminum bottom can lead to burn-through. The original charcoal grate was made of cast iron. I wonder if that was on purpose to increase radiant heat. It also had two round shields to try and protect the two vents from ashes. The PK vents are completely separated (good!) so you can open one (under charcoal on one side) and close the other (under the food with no charcoal below) to do indirect bbq.
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The good news about the grate is that you can buy a regular grill grate meant for the medium sized Big Green Egg which will fit perfectly. You can buy stainless, but I went with cast iron to be closer to original. I also added two cast iron coasters/trivets to substitute for the original grate's shields:
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My grill didn't have the bottom vents. You are right that the "new" PK company isn't currently interested in producing or carrying parts for the Duchess. They did refer me to someone who makes stainless replacements, though. I primed the outside side of his nice vent covers and then painted with ulra high heat silver to make them look closer to original. (No paint on the side facing in to the grill!)
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For the top cooking grate, I am where you are. The old nickel steel is hard to make decent after years of neglect, and it is not a regular size. You can buy a custom carbon steel grate from The Burn Shop:
Custom Grill Grates
If you are ready to purchase a custom grate, please fill out the order form and in the description please include the diameter/dimensions of the grate you are wanting. Also describe the design you are wanting or include an image in the file attachment. We will confirmtheburnshopwf.com
I am planning on contacting Mark Barron of Killa Grilla about making a stainless steel replacement with the same fold-up side as the original. I am hoping he will be interested and look at this as a new product not a one-off. Mark's thick stainless will make fine grill marks for high heat, but that design lets you switch to indirect low and slow as well.
Here is a picture of the outside of mine after cleaning up. Not too bad for a grill as old as me (60). I opted to only clean the old powder coated finish. I could have spray painted, but I will just wait and see if someday I can get the powder coating redone. It is a much better finish than you can get from rattle can paint:
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Once I get my current top grate at least adequate to cook on, I am going to try this thing out! I hope to do a comparison between the PK Duchess and a Weber 18" kettle.
Did you ever happen to hear back from Mark @killagrilla?