Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:5078746.
V2 EDIT: Ok so I went back in and modified some things. I looked up some formulas for designing ocarinas, and used those to change the total volume and the size of the embouchure hole. Now with all the tone holes and the ends closed, it should do C5. In the process I made it shorter and thicker. I also changed the locations of the tone holes a bit, but not their size, or in any accordance with what note they produce.
Questions: Are you supposed to be able to completely close off the ends with your thumbs? I'm not sure that you can with this version, it might be too wide. Do the calculations for an ocarina of equivalent volume work for a koudi? Would the calculations for a normal flute be better, or worse?
I didn't see any designs for a koudi (口笛) on here, so I tried throwing one together myself. This is just a rough draft to see if it even works at all, so please forgive the callous disregard for proper dimensions. It might be terribly inconvenient to hold properly, and it isn't in any kind of tune at all. I also tried to put more of a quena style labium on it because I just like that. The bore also tapers just slightly towards the very ends.
If it works, I might try to refine it by: Using a more appropriate length to bore ratio. Positioning the tone holes more ergonomically. Sizing the embouchure hole and tone holes correctly to put it in tune. Giving it more of a conical bore, in both directions just to see what that does.