Talkbass is a good resource for bass cab building, if not much else. Pay attention to the responses from Bill Fitzmaurice.
You're not going to find plans for a 215 for bass because the cab dimensions will depend on the drivers you're loading it with. Electro-Voice put out a bunch of plans for cabs for their speakers - the TL806 is the most well-known of these, but I think they have plans for a 215. There are a bunch of modern cabs that are basically clones of EV's old designs.
Hardwood looks good, but it's weaker and more resonant than good plywood. If you want that furniture look, you might want to look into veneering.
Get the latest version of WinISD from their Facebook page and learn how to use it. Pay particular attention to the 'maximum power' and 'rear port velocity' graphs.
Front-load your speakers so you can cross-brace the hell out of it. This prevents vibration, which is just energy being wasted as movement when it could be instead used for MOAR LOUD.
Actually, come to think of it, if you want plans check out these links:
http://www.billfitzmaurice.com/Simplexx.html (paid)
http://greenboy.us/fEARful/DL/ (free)
...just understand that those plans were designed to work with particular speakers, and may not work well for other speakers.
For guitar, just toss speakers in a box. Doesn't even need a back.
