Can I make a cover for my flue?

It is not lawful to manufacture or fit an after-market flue cover.
Firstly the flue must be fitted to the manufacturers instructions (see Clause 8.6).
The addition of a flue cover is considered a modification by the Technical Regulators.
Caravan appliances are “”Type A”” certified so no modification is allowed (even to the installation instructions). Secondly the Technical Regulators agree this changes the appliance to a “”stowed appliance”” and as such the cover requires an interlock to prevent gas flow when the appliance is stowed (see Clause 6.3.2).
If you intend to do this, plans must be submitted to the Gas Technical Regulator for approval. And you must prove it is safer than the “”deemed to satisfy”” clause.