This may be clear as mud, so don't hesitate to ask for clarification.
I've made 5 or 6 of these and to give you a comfort level, and tell you that you don't have to over think this thing, they only take about 2 hours to build.
First, make the lid from 3/4 MDF. MDF is dimensionally strong, easy to machine and flat. If you look closely under this lid, you'll see that the sealing groove has had some modification. The top of this particular drum has a flat spot and it's easier to rout out the groove than straighten the steel ring. Trash cans are usually round when new. A 2X2 piece of MDF was used for this lid, Set the lid (centered as close as your finger tip allow) draw a line around the underside. Flip the lid over; find the center of the just drawn circle. Exact is not really critical. Drill a VERTICAL hole for your circle jig's pivot. Select the appropriate sized core box router bit for your drum's/can's lip. Set the circle jig's radius to cut a groove for the lip. Rout it.
You can leave the top square or cut it round with a BS or router. If you use a router, set the top on a piece of rigid foam insulation. You should already have that anyway. I use a 1/2" straight bit for cutting pieces off. I use the angled hose connectors (around $5) and a hole saw to make the vac holes. First one uses the same starter hole as the circle jig. The second is located where the hole is about 1 inch from inside of the rim slot.
If you are using a drum, you can size the baffle from the inside edge of the lip groove. With a tapered sided you'll need to make an inside story stick. 2 small cutoffs, pointed/beveled on the ends that contact the barrel, each a little smaller than the inside diameter of the barrel but when spring clamped at 2 places will reach the opposite sides. Check at 3-4 places. I've used 1/4" hardboard for the last 4-5 baffles. Lighter weight, works fine. Cut out a circle based upon the story stick. Clamp that, centered to the lid and bore 3 holes thru both for the mounting bolts. View pic for locations.
Next you want to remove about 1 inch of the baffle edge for 270 degrees. Use a compass and mark up a hexagon. Small compass...small hexagon, and extend the lines. Leave the area between 2 of the lines uncut. Whether the uncut area touches the can side, or just comes close is not critical. Here's how to draw a hexagon.
http://upload.wikimedia.org/wikipedia/commons/a/ab/Regular_Hexagon_Inscribed_in_a_Circle_240px.gif
Here's a pic that should show most relationships when finished.
The last PVC piece is not glued so I can change it's angle. This alignment works best on this drum, but on a trash can, I have to change it slightly.
