Based on what I've read at diyaudio, I wouldn't recommend using solid wood (especially 25mm thick). It has to do with the resonance of the box in reaction to the mated drivers. Depending on the design of the enclosure. sometimes it is even intentional to have thin baffles such as some Harbeth enclosures and Zigmahornet enclosures, in which the design lets the box itself resonate well with the drivers.
I recommend high quality 15-18mm plywood for your encloure. If you really want solid wood included in your enclosure, you could accent your speakers with side wood panels na lang, like some Sonus Faber enclosures.
