They will work good for you. They are very thin, so you won't feel them. Happy to help.

Thank you! I looked at that website, and it looks like it has what I'm looking for. I have two dulcimers that lack inlays, so a sheet of 20 will serve me well.

I have used those a few times on various instruments. My latest was on a custom Folkcraft dulcimer that had all the fretboard inlays I'm used to, except it didn't have one on the 5th. My eyes are just used to one on the 5th, all my other dulcimers have one there. So I got the white pearl stickers from inlaystickers.com, and put one on the 5th. Can't tell the difference between the inlays and the inlay sticker. 

Go directly to their website for many more choices. Still free shipping. 

updated by @susie: 03/10/20 09:23:00PM

I haven't used them, but a couple of my guitar player friends use them for marking certain chords in unusual tunings.

I'm enjoying a dulcimer I added to my collection, but I'm a bit thrown by its lack of fretboard markers, which I'm used to.

Have you used something like this and if so, has it been useful? Is there a better alternative? 

https://www.amazon.com/dp/B00PGQYQJY?tag=duckduckgo-brave-20&linkCode=osi&th=1&psc=1