Adding material like you're thinking is risky because the thickness and type of material is already an integral part of the design. Your safest/best bet is to simply use quality craftsmanship in your joinery. If you are going to add or reinforce the safest least risky place to do add is the baffle. The vast majority of energy comes from the driver and you can increase the baffle by adding material to the outside which will not affect interior cabinet volume.
The other stuff you're talking about will add a lot of weight but not affect the sound a whole lot one way or the other. Oh it will have an effect. For sure. Just not one anyone is likely to notice in your anticipated environment and use.
This isn't really my area but I've always been of the impression the keyboard sound that sucks isn't so much the amp or speakers as the pre-amp or sound card. Whatever it is on a keyboard that generates the signal. You could check this easily enough simply connecting a guitar amp you like to your keyboard. If it still sucks, its not the amp. Its the keyboard.
The other stuff you're talking about will add a lot of weight but not affect the sound a whole lot one way or the other. Oh it will have an effect. For sure. Just not one anyone is likely to notice in your anticipated environment and use.
This isn't really my area but I've always been of the impression the keyboard sound that sucks isn't so much the amp or speakers as the pre-amp or sound card. Whatever it is on a keyboard that generates the signal. You could check this easily enough simply connecting a guitar amp you like to your keyboard. If it still sucks, its not the amp. Its the keyboard.