Maybe that is why it wa ssimplistic, he was describing a way to locate the steady, clearance in the middle would be upto the builder. Yes you would need to cut a clearance notch or similar for the feed screw just like the third hole if going for the drilled/bored/reamed option.
Or two small blocks that bear on the outside of the bars would do the same job, screwed on with clearance holes they could be finally tightened up in position so reducing the need for accurate sizing of a central block. Infact given the OPs limited equipment just a single block at say the front would do, so long as you pushed that up against the front bar when setting and again when positioning the steady and leaving the middle clear for clamping bolts. Graver rest would not even need a block, just a flat bar will do and the added ability to swivel it’s position on the bed bars would be a bonus as you would get more versatile positioning of the rest, similar to the banjo arrangement found on woodturning rests.