The raised nut for converting to slide guitar is a readily available chrome plated accessory sold at many music stores. It is a flat plate bent to an acute angle allowing the ends of the plate to rest upon the head stock behind the nut and the fingerboard in front of the nut while supporting the strings in small spacing notches. I paid I think about $0.80 for each of the ones I carry in my case. They can be installed or removed in a minute or two. With a "round neck" guitar optional "finger style" is only minutes away.
Some resonated guitars builders use a felt (as from a vintage mens dress hat) "shim" under the cone. One could be purchased directly from one of these builders as a replacement part. Or, any type "cordage" from a hobby/sewing shop could be tacked about the edge of the hole. This "gasket" is only intended to subdue the risk of probable vibrations of a poorly fitted cone in cheaper instruments. It is not really needed if a good fit is obtained.
My first "resonated" guitar was a Sears catalogue Silvertone (I think made by Gibson) circa 1940's with purchased accessory raised nut and an inverted paint can lid under the bridge.
Very good guitar buys abound when a "neck reset" becomes required. Good cheap place to start for this project. Delete the "finger style" option.
Hope this helps widen the possibilities for you.