I am using readymade plastic cabinate, should I replace it with metal one?

Yes, You should go for a metal enclosure. Approx 1mm thick Galvanised [ Zinc Coated ] Mild Steel is most ideal material for enclosure.

Keep the box big enough to take care of heat.

The cover of the enclosure should be fitted with multiple screws and not just four screws on the corners. 2 inch pitch is ideal for screws on cover.

Don't keep larger holes or openings on your enclosuer try to keep the openings for wires etc as small as possible. If at all larger holes are needed for things like LCD display etc. Use the clean box [Main ciruit ] - Dirty box [ LCD ] method.

The theory which works behind metal [ Steel ] enclosure is given below.
EMI consists of two things Electrical charge and Magnetic Field.
1.Using an enclosure made up of Conducting material [Any metal Cu, Al, Fe etc ] you get rid of Electrical part.
2. To get rid of magnetic part you need to disspate the magnetic emergy by eddy current. Thus you need a ferromagnetic material such as steel.

Say, If you use an Alumunium enclosure you will get rid of elecrical charge but magnetic interference still exist.
Thus Mild steel with Zn coating is chosen for enclosures.

Regards,
Prahlad Purohit