How do I get a plywood window insert to actually seal properly?

The draft you're feeling around the edges of that eucalyptus board is ussually caused by the 9mm plywood flexing slightly when you tighten those wing knobs. Even with D-profile weather strips, plywood that thin tends to bow in the middle, especally if your clamping points are only at the corners. You can try switching to a **closed-cell EPDM foam tape** instead of the standard rubber D-profile. EPDM is much denser and doesn't compress as easily, so it creates a better gasket when you've got a slightly uneven surface.

Another trick for those European tilt windows is to use longer bolts for your butterfly nuts so you can add a backing bar. If you just clamp against the frame, you're only sealing where the bolt sits. Putting a small strip of scrap wood on the inside of the frame that the bolt passes thru will distribute the pressure more evenly across the seal. It stops the board from pulling away in the center where the draft is likely sneaking through.

Don't worry about the wonky handle placement; it actualy makes it easier to tell wich side is up when you're lugging a heavy sheet of multiplex onto a window sill. For the negative pressure issue, you're never going to get a single-hose unit to stop pulling air from the rest of the house. That's just how the physics of those machines work. They exhaust air out the window, so that air has to be replaced from somewhere—usually under your door or through light fixtures.

If the window seal itself is whistling or feeling cold, check the corners of your plywood. When you have the hardware store cut it, they usually leave a sharp 90-degree edge that can pinch the weather stripping rather than compressing it. A quick sanding of the inner edges where the board meets the seal can help the foam sit flatter and close those tiny gaps that the D-profile misses.

Have you ruled out the actual hose-to-board connection point? I've seen people spend hours on the perimeter seal only to realize the plastic flange on the hose isn't sitting flush against the plywood. Also, are you sure that 9mm eucalyptus isn't just porous enought to be the culprit? It’s pretty thin stuff. If you're feeling a breeze, it might not even be a gap in your weather stripping—it coud be the negative pressure pulling air trough the wood's grain or a small crack near where you cut the vent hole. Have you tried a candle smoke test specifically around the flange and those wonky handle holes you mentioned?

Spot on about the pressure distribution with that thin board. I had a similar setup with a tilt-turn window and the backing bar relly is the way to go to stop that bowing. One thing people ussually overlook though is the actual wood finish. Eucalyptus plywood is pretty moisture-sensitive, and if it's raw, it'll warp just from the humidity of the exhaust air. I'd defintely hit the edges and the back with a quick coat of sealant or even some leftover exterior paint. If the wood swells even a tiny bit, it'll ruin that tight seal you're working so hard to get with the EPDM tape.

since you mentioned that your drilling for the handle was imprecise, it is highly probable that you have neglected to seal the actual bolt holes where the hardware passes through the plywood, as even a fraction of a millimeter of clearance around a fastener can act as a localized induction point for unconditioned outside air due to the negative pressure differential.

i woud suggest retrofitting those mounting points with some small neoprene washers or a dab of removable silicone sealant to ensure that your wonky handle isn't actualy acting as a miniature vent, especially since eucalyptus is a relatively soft hardwood that tends to compress unevenly under the specific torque of those wing knobs.