Okay gotcha,
One can't cut HALF a hole with a hole saw so just tack a temporary plate to the bottom of the frame the same thickness as the frame. This will give the hole saw a complete 360* cutting surface. Once through the vertical part of the frame rail then the flanges can be cut with a hacksaw or cut off wheel. Easy peasy. With a really large hole saw one needs to slow the rpm way down to keep from burning the cutting teeth. The old nasty black sulfur based cutting oil is best for this.
