A box is better than simple flush mounts. Nearly any material is good, you want something that won't vibrate. Thick, 3/4 or better wood is good, much thinner and it will vibrate and 'pollute' the sound.
Another advantage is that they can be moved around, cockpit or deck etc.
Many companies make 'bookshelf' speakers that sound good, many others make housings for a reasonable cost.
I'm not an audiophile, but really like clean sounding music.(and also have a wide range of tastes from (old 'real') country, classic rock, new rock, classical etc. It's normal to hear Tanya tucker followed by led zep followed by metallica followed by ???) Consequently I have a 600 watt (RMS not 'peak') system that is seldom turned up loud enough to interfere with conversation, simply because it provides a great clean wide sound range.