Make a flat bit
For my sins I just went to B&Q for a bit of steel and ended up with 1 metre of 10mm(ish) threaded rod as it was the biggest solid rod that they had that would still fit my drill chuck. I used my plumbing blowtorch to heat it and after a bit of hammering, grinding and tempering it looked like this.
A bit rough but I had a go at drilling a hardwood log that was about 3" thick and it made light work of that. So, a good result and easier than I thought it would be. Also an awful lot cheaper!