Identify the equipment before designing the cover
A boiler, consumer unit, gas meter and water stop tap all need different amounts of access and ventilation. Start with the exact product instructions, and get the right qualified trade to check anything you are unsure about before you cover it.
Design for the person who must service it
A cover should open far enough to inspect, isolate, repair and replace the equipment. Removable panels, doors that open fully and clear service zones are more important than making every seam disappear.
Keep labels and controls available
Do not hide warning labels, isolation controls, meters or fault indicators in a way that delays access. If the enclosure changes airflow, heat or combustion conditions, stop and get professional advice.
Treat the joinery as cosmetic only
The enclosure can make the room look better, but it does not change what is happening behind it. If gas or fixed electrics need altering, use the right qualified trade.