I played around with drawing one up using a 60 degree dovetail cutter (because that's what I have) and to get everything to come out right the cube size was bigger than any brass and aluminum stock I have on hand. So, now that I have my shaper I'm going at it again because I can make any angle/size dovetail I want.
I've got some 1-7/8" dia. bar so the biggest square I can make is around 1-5/16". With a 75 degree dovetail I can get things to come out alright. The challenging part is cutting the dovetails to the correct size, angles and location so they all line up, since there are three of them and they all have to be right for nice tight fits. I've got a plan I'm going to try using dowels to measure size and placement. Should be a fun project.
From playing around with the design in Fusion 360, you'll need to size things depending on your method of cutting the dovetail and, if you use a mill cutter, the size/angle of the cutter. In the video you'll see they use a pretty steep angled cutter.
Draw it out in CAD or paper and see what you come up with.
If I remember correctly (i's been about 55 years,) I used a piece of about 2 1/4" mahogany, cut in to two pieces equal length. I bandsawed the cavity in one piece the dovetail in the other. Both parallel the sides. Then glued it together. There was some fitting involved, but that 's not part of this discussion. Once the glue was cured (overnight) I turned the tablesaw blade to 45º and made two cuts, returned the blade to 0 and cut the other two sides. It looks just like your example except it's 6" long.