After much research and experimentation I was unable to find any soundhole cover that was a perfect fit for my 000-18. Ultimately, I stumbled upon the Maton guitars soundhole plug made specifically for the Maton 808 Tommy Emmanuel model acoustic. I ordered one and found it was a hair too small to snugly fit my Martin the lip did prevent it from falling in the hole. What I did next was to take black electrical tape and wind it around the outer edge until I built up enough width to create a peer seal. I trimmed off the extra tape and all seemed good, or so I thought. Overnight, the tape contracted and slipped off the soundhole cover. I re-applied the tape but this happened several times. Ultimately, I took the ring of tape off and stretched it out so I could slip it back on and then drilled small holes along the edge and then put small screws through each hole and secured them with crazy glue. Next, I took heavy duty velcro and attached my iRig Acoustic Stage pickup to the back of the cover threaded the cable out through the front. The result? Perfecto! Now I can gig live at high volume without feedback. I'd still like to find some place that could make me a custom cover, but in the meantime, this does the trick.