furoshiki sizing question

I've read that the diagonal size of a furoshiki should be 3x the longest side of the object. But for a box, that seems to ignore the height. (Please excuse all the math that's coming...)

I have a box that's 8" square, and 4" high. So, 24" total required to wrap completely around, from bottom to top.

A 50cm (19.7") furoshiki has a 71cm (27.8") diagonal. By the "longest side" method, that's big enough for an 8" box.

But if you do the math, 27.8" minus 24" leaves only 1.9" at each corner of the furoshiki with which to tie the knot. That seems like a very small knot. A 70mm furoshiki would leave 7.5" inches at each corner for knot-tying, which seems like it would result in a more festively sized knot.

Am I missing something? (From what I've googled up, the 70mm size is more traditional for a bento box. My box is larger than a traditional bento, I think - at 8" x 8" x 4".)

Also, any links to (U.S.) Amazon items that are the 70mm size would be appreciated... Several searches have turned up very few options, and as this is a New Year's gift, I kind of need the two-day shipping.

I guess the wrapping method where g3 times the diagonal lengthch is recommended is the first one on this page, meaning, not making a knot, but simply wrapping the box. (In this case, you might just carry the wrapped box, or put it in another outer bag for carrying, and use the gfuroshikih as the inner wrapping, to be removed only immediately before you present it to the recipient.

https://www.furoshiki-kyoto.com/how_to/Lecture_howtowrap.html

you can't ignore the thickness. it depends on the ratio of the longest side and the thickness.