These are all great ideas - many of which I've done in the past, some new to me. Most of them reflect the "craftsman" approach, but neglect that the OP (probably) isn't that type of builder - or he wouldn't need to ask in the first place.

Not many new builders have 5/8" brass tube on hand - heck, many hobby shops don't, either. The OP wants to drill a 5/8" hole through 1" thick balsa. He can stop in at nearly any hardware/home improvement store and get a SHARP forstner bit for less than 10 dollars and be done with it. These other suggestions won't save him substantial money (if any - priced brass tubing lately?) and will cost him time.

Balsa is not foam. TRY sharpening a piece of 5/8" brass tubing and twisting it through a piece of 1" thick balsa by hand - across the grain, not with it. I once did a 3/8" hole in a 1/2" thick block that way - NEVER AGAIN.