Leaderboard

Ken's right. A lot of old dulcimers had concave fretboards lowest around the 7th fret. Idea was that strings vibrate in an arc like a jump-rope and the spot half way between the two ends is the lowest part of the arc...

Well, a planer is the perfect (but expensive) solution. First, arrange to cut your fretboard so that the grain is at right angles to the future top, not flat-sawn with gran lines that parallel the top. Barring a planer or hand planes and skill using them, you can make a flat sander. I made mine from an inexpensive 3/4" x 6" x 36" replacement marble window sill from one of the Big Box stores. To that I glued two (different grits)) cut-open 3x24 belt sander belts (grit side up, of course).