Metal would be a good choice, but it will not do much to reduce any noise and to look anywhere half decent you would need access to a workshop and have experience in the use of relevant tools.
Believe it or not, wood material thicker than 6mm is quite flame resistant.
If I were to box in a boiler, I would consider a kitchen unit of the correct size – modified with no back and additional holes for any pipe-work.
Make sure that the enclosure has the minimum spacing required by the boiler installation, or be prepared to dismantle the kitchen unit each time the boiler needs servicing.
Besides modifying the enclosure to suit, you will need to ensure that the enclosure fixing to the wall is achievable with the boiler in situ.
I have recently installed three Ikea wall mounted units in my kitchen, they do a wide variety of sizes – I suggest you take a look.