1/2" to 3/4" particle board or plywood would work and can be found pretty cheaply at HD or other large home improvement stores. Best yet would be MDF which is what many of the most exotic / expensive floor standing and bookshelf speakers are constructed from while veneering the exterior with hardwoods or other surfaces for cosmetics. It offers the most consistent density and provides ideal dampening qualities.
As far as the advise to not build enclosures behind ceiling or in wall flush mount speakers, it really doesn't matter if they are $50 or $500 versions. The crucial thing is knowing what size of an enclosure to build to work with the bass driver "woofer" the speaker utilizes.
If you have ever gotten into building speakers, you know that when you buy a bass driver, it will come with a list of specifications.
Calculations from these specifications will let you know the best internal volume the enclosure should be for that driver to work at it's best. If using a ported design, it will let you know the length and diameter the port needs to be. Rarely are these specifications given with flush mount or ceiling mount speakers. I have never seen them with speakers in the mid to lower price points.
There are test procedures that can help you to obtain the specifications of a driver.http://www.bcae1.com/spboxad3.htm
The main thing is they design open backed speakers such as these to be of a "Free Air" design allowing them to work fairly well with a wide variance of enclosure space behind them. Be it 2 cu. ft. in a short wall to 1000 cu. ft. of attic space, insulated or non insulated. So the advise often given to not build enclosures is based on the fact that without knowing the ideal size of an enclosure to build with the driver your working with, you may get less that desired results than without the enclosure. With many installation locations, you have no option of altering the enclosure once drywall is installed.