Best products for baffle material s ?

The best material would have extreme tensile strength and Young's modulus of elasticity, but also high internal damping. Metals can of course have high strength, but very poor damping. And if you search, one can find tables which list the Rockwell Hardness for many different species of hardwood, some can be quite high. And wood of course has high damping. Metals do have the advantage of mass, which means more energy needed to excite resonances, but lack of damping means energy storage or 'ringing'.

Wood in it's many forms is a good compromise. MDF however is typically too low in terms of strength. But there are some high-density ultra-refined MDF variants which are better.

Of the modern materials available today, thermoplastics such as phenolics I would have to say are best. They come in 1000 forms, unfortunately most are very expensive. Spec sheets are also available which list the parameters mentioned above. But some can combine tensile strength higher than 6061 aluminum, AND excellent internal damping.

Chayro - you have things completely backward. It's building resonant, colored cabinets that is completely trial-and-error, and based on misguided theory. Strong, massive, resonance free cabinets ARE better.

The job is to create an instrument as true to the source as possible. And cabinet resonances, intentional or not, are pure 100% distortion that are delayed in time and uncontrollable in relation to volume.

If the designer intends to create a colored/innacurate music reproducer, there are ways one can do that in a controlled and predictable way. Cabinet resonances are not. The reality is that some companies choose to make resonant boxes not for performance reasons but budget reason, they are always much cheaper and much simpler to build. They then tell the marketing dept to come up with ways to obfuscate the truth.

Chayro - I agree cabinet design is more complex, but he is at least asking the question. And I have vast experience I'm willing to share. So by the end, he'll have some knowledge needed to make good decisions. Would I say that Harbeth and Spendor are based on misguided theories? For the most-part, no but purposefully introducing energy storage and uncontrolled resonances to introduce some 'color', absolutely is a misguided approach. Color can be added without all negatives of a resonant box, reality is it costs money and time, and it's easier/cheaper to let the marketing dept try and convince people that it is actually a good thing.

Regarding baffle dimensions, it's the size of the baffle, position of the tweeter on said baffle, and shape of baffle edges (square, chamfer, roundover, and size of chamfer or roundover) that determine the diffraction signature. Typically with less than a 1" roundover there is a 2-3db diffraction dip around 3000hz. This dip is often accounted for in the crossover by adjusting the knee to bring up the diffraction suckout area. That's broad strokes, but gives a good general idea of what's important. Much better than adjusting the crossover to bring up the diffraction area is to build a baffle/cabinet that eliminates the destructive point-source diffraction. If the designer of the speakers you're building says no account was made in the crossover for the diffraction dip, you can incorporate large roundovers as long as you keep the baffle dimensions within an inch of intended. Because the baffle dimensions also impact the boundary reinforcement of the tweeter, which impacts it's bottom end extension. FYI, the area below the woofer on the baffle is of minimal importance. So if you keep the tweeter located where intended on the baffle (center, left, right), baffle width the same, and distance from top edge of the baffle to the tweeter all the same, then you can for instance make it into a floorstander and keep the tweeter response the same. That can have a slight impact on bass, typically means more reinforcement or more bass. But you're only talking 1/2db to 1db at most.

Ptss - you'd be surprised how much energy even small drivers can dump into a cabinet. The magnet systems are extremely efficient, where drivers are much less efficient is coupling to air. But yes, even with smaller drivers cabinet construction can be critical.