Dale Saukerson wrote:
The size of the box does change the characteristics of the bass notes. But many many people have put 6x9 speakers in nominal noncalculated size cabinets without regard or complaint about the sound. It is a common way to DIY a set of shop or garage speakers. An example size would be 12 inch tall x 8 inch wide x 6 to 8 inches deep
+1
Please don't use an open back enclosure. You'll wonder why you even did the project. Any type of sealed enclosure will be better than that even if it's not much bigger than the speakers themselves. However I do recommend you use a size at least as big as what Dale is recommending to have decent sound at all. Those 6 x 9 auto speakers are capable of really good reproduction when implemented. correctly.
It's important to remember that good speakers are often referred to as a system. The driver is just one part of it but equally important if not more important are the components that cause the driver to work to its best advantage. This is why an open box is pretty much worthless except as a driver holdet.
Here's a suggestion. When you get your stereo pair built, temporarily leave the back off of one so you can compare the sound with the sealed box. Then let us know how satisfactory you think the open box sounds.
P S. If you're going to go to all that much trouble anyway, use as thick wood for the box walls as you can, use some internal bracing that is offset from midpoints, and fill the box with inexpensive pillow stuffing that you can get at the fabric store. Those three techniques can reduce unwanted and annoying resonances that tire the ears.