[Confuzed]

I got to this late but you can lay a coat of laquer between the spanish cedar and oak. this will seal the oak. You don't want to do this to your spanish cedar..it would be nasty.

Everything else has been answered really.. If you build one in the future, check your local lumber mills. They will have a larger variety and you won't have to pay department store prices.

You can save money by using a cheaper wood for the outside box and lay veneer over it..still line with spanish cedar or another mahogany, though. You could buy some exotic wood, have your friend use a bandsaw to cut several thin planks and then joint/plane them..this will allow you to save money and use any wood you want for the exterior without breaking the bank.

Looks like it's turning out well. Nice project.