Hi @kitkat20,
In the lower section of the cabinet, are they doors at the back? Is there any timber framing around them?
For that bottom section, I would suggest adding some kind of backing panel to cover up the seams. A sheet of plasterboard would be a relatively simple solution. You could screw it to the frame using plasterboard screws, then cover the screws with Multi-Purpose Joint Compound.
For the upper section and the seams around the edges, you could plaster the corners using the same multi-purpose joint compound and Paper Jointing Tape. Have a look at 3:15 in the video How To Install Plasterboard for a basic guide on plastering internal corners.
Once everything is plastered, you can give it a sand with a sanding sponge, then prime with Dulux 1Step Prep Primer, Sealer & Undercoat and paint with Dulux Wash&Wear in a colour and sheen of your choice.
With the ventilation, I would be hesitant to seal it up without knowing why it is there. Generally, if there is a vent, it is because there is a need for ventilation. Closing it up could cause moisture issues, which can lead to mould.
If you were 100% confident that closing it up wouldn't be an issue; you could plaster over it, using a suitably sized plaster patch as a backing. You could use the same multi-purpose joint compound as before, sand it with the same sanding sponge, then prime and paint it like above.
Let me know what you think and if you have further questions, please don't hesitate to ask. I am more than happy to help.
Jacob
