Electronic – Securing a PCB board to a plastic casing – can I just glue the bottom

You will have a hard time achieving 1 nF by just laying out copper on a standard two-layer FR-4 board. The capacitance is given roughly by the parallel plate equation:

\$ C = \dfrac{ \epsilon A }{ d } \$

In this case

\$ C = \dfrac{(4.7)(8.854 \times 10^{-12}) A }{ (1.6 \times 10^{-3} ) } \$

or

\$ C = A (2.6 \times 10^{-8}\mathrm{F/m^2}) \$

Meaning you'd need .038 m² or 380 cm² of copper area to achieve 1 nF. I used 4.7 as a typical dielectric constant (relative permittivity) for FR-4 and 1.6 mm as a typical board thickness.

It is not uncommon to make pF scale capacitors by parallel copper regions, but it's normally done in multilayer boards where the d term can be much smaller. This kind of constructed capacitor can achieve lower ESR and ESL than a discrete capacitor, so it is valuable for bypassing power supplies in very high frequency circuits.

There are also companies that make special materials that can be laminated up in a multilayer PCB to provide a high-dielectric-constant layer, enabling construction of even larger capacitor value by metal patterning. 3M is one. These are often called embedded capacitors or buried capacitors. Contact your PCB fabrication shop to see if they support this type of material.

To the extent that your aluminium box doesn't have large holes (relative to wavelength) in it, it will act as a Faraday cage and significantly attenuate any electromagnetic radiation passing between the inside and the outside.

Sometimes, this is what you want. If you are trying to reduce unintentional emissions of your circuit, or reduce noise received from the outside, this is good.

Sometimes, this is not what you want. If your PCB includes an antenna, you probably want to radiate electromagnetic energy, so enclosing it in a conductive box is probably counterproductive.