This one is a bit tougher on the loom, so it really needs a wooden loom, instead of card.
Step 1: Making the loom.
I used a hole-saw to cut the disc, and re-learned an important lesson - you get what you pay for.
The hole-saw cost a whole British pound (less that two dollars). The centre-bit didn't drill through the ply, it burned it's way through, because the bit was already blunt when it came out of the packet. As soon as the saw-blade reached the wood, the whole thing locked, spinning the ply out of my hand. When I clamped the ply, and tried again, it spun the drill out of my hand!
Going slowly, I eventually wore through the ply. If you look at the photos, you can see that the edge of the hole has a 45o chamfer on it, because the blade vibrated so wildly.
Anyhoo, I eventually produced a ragged-edged disc, and threw the hole-saw in the bin (recycling bin, though, so it's not a total waste...)
Using my rotary tool, I cut sixteen equally-spaced slots around the edge, measured with a paper protractor (see the next step), and smoothed off the edges and central hole to make it more comfortable to hold, and to prevent the yarn snagging.
I then used a coat of sanding sealer to keep out skin-grease, and to show up the plywood's various grains. If you use sealer or varnish on yours, run a scrap of paper along each slot to stop stray drops drying in the wrong place and blocking the slots.