Louie,
Depending on the size of hole you want, you CAN make a punch of brass not copper tubing. Brass is harder than copper...
Way it works - shave the inside of the tube end to form a cutting edge while the outside remains stock. An old #11 blade works just fine...
This is good, depending on the sizes of brass tubing you have, or are willing to buy for this use. Say the largest you find practical is about 5/16" diameter... You use such a punch by pressing and rotating the cutting edge, with the stock to be 'punched' backed up by a cutting mat or a piece of carboard like the back of a lined pad.
If you press into the balsa straight enough, you'll get a very neat hole that size with no splitting or tearing. Of course, the 'plug' you punched out has to be popped out of the punch each time. No problem; a piece of, say, 1/8 music wire works well.
You could use smaller punched holes like this for the corners of a larger area cut-out. Figure where you want the shape, cut the corners, and slice out the area between them witha (sharp) #11 blade. Again, it helps to do this over a backing, and to guide the X-Acto on a metal straight edge.