Decent vent covers are surprisingly hard to find! Either they're cheap stamped steel which rattle anytime the fan runs, or they're nice looking, wooden, and hideously expensive. My old house has a few extra long vent openings too, which just made things more difficult. So, I fired up the printer instead.

The model is broken up into three main parts: left, right, and inner. Each part has two objects, a grille made from infill and a frame. The left and right parts can be joined together or they can be joined to any number of inner parts to create longer vent covers.

Printing

Each part has a rounded top-front edge and a flat bottom edge. The left and right parts are just under 205mm each and the inner part is just over 195mm. So a 4-panel grille (left + 2 inner + right) will be 800mm (~31.5in) and a 2-panel grille (left + right) will be 410mm (~16in).

PLA worked well for me, though if you're putting these in a room that gets a lot of direct sunlight or have a chaotic household then ASA or PETG may be a better choice.

The grill panels are separate objects within each part, so changing the infill pattern is easy. I used crosshatch as it provides good airflow and hides the vent opening well, but grid, rectilinear, triangle, tri-hexagon, gyroid, honeycomb, and 3D honeycomb will work as well. To change the pattern, go to the Objects tab in the Process pane, then for each part select the Grille object and adjust the Sparse Infill settings.

Note: The grille pattern is only visible after slicing.

Finishing

The parts can be joined using a cyanoacrylate glue. Remember to triple-check each part's orientation before gluing them together! There are no built-in alignment connectors, so it helps to use a couple of thin, flat-ish pieces of wood (eg: paint stir sticks) and a clamp to hold the parts in place while the glue sets. Be careful to not over tighten the clamp or the glue may set with the part slightly warped.

When using well-calibrated filament, the layer lines will not be noticeable once mounted. If you often have trouble with under/over-extrusion, stringing, etc, then smoothing and/or painting will help ensure each part looks consistent. Just take care to remove as little material as possible, particularly on the rounded edges. If you decide to paint them, I would recommend using spray paint and changing the grilles to use a simple infill pattern like triangle or rectilinear to help ensure consistent coverage.

Mounting

Once assembled, M4 screws can be used to mount the grille. The screw holes are not chamfered, so if you're using flat topped screws be careful not to over-torque them or you may split the part. Alternatively, a strong double sided tape or a plastic-compatible construction glue (eg: polypropylene adhesive) can be used. Paintable silicone can be used to create a smooth join between the grille and the wall, the same as you would for baseboard trim.

The printable plugs can be used to fill in the screw holes. They're a very snug, flush mounted fit though, so be prepared to leave them in permanently! If screws are used to mount the grille, then the plug height will need to be reduced. If you're unsure how tall the screw heads are, reduce the plug's height to 4mm and try to avoid inserting them too far.