There are no plans for cardboard dulcimers. There are only kits to put together or completed cardboard instruments. The kits are under $100.
You can easily make a trapezoid or box dulcimer body roughly 30" long by 5" to 8" wide by 2" deep using tape and Elmers glue. If you want anything fancier, I suggest you buy a cardboard kit.
A length of straight 1x2 pine or poplar or maple from one of the big box stores is your fretboard for only a couple dollars.
A set of inexpensive tuners is under $20 and fretwire is under $7 from CBGitty or StewMac.
A set of strings under $8 usually from any music shop.
The spacing for the frets you can get from WFret or the StewMac calculator or a couple other places for free.
If you'd like any help in the construction process, join us here in the Groups area under Building Dulcimers, and we can easily talk you through a build...
updated by @ken-hulme: 12/29/20 03:48:57PM