The design of a planar subwoofer is not too complicated like a subwoofer box or reflex bass in general but in order to get a satisfying sound character the design must be precise and use good materials especially for speaker boxes. Planar itself has 2 types namely type long and type short. Materials that are expensive are not necessarily good when talking about speaker boxes; for example plywood which costs more than MDF material it turns out the results are not as good as MDF material if applied to speaker boxes. Meanwhile particle is a salon material that is sold in the market but particles are not good ingredients and the quality of the results is still under plywood.
But you can also choose wood materials such as meranti which of course is better and stronger than the ingredients mentioned above. So if I may suggest choose fiber or MDF because this material is better than plywood and easier to make than wood. If you can or it is very possible if you make a box from wood then it doesn't hurt because wood is a strong and suitable material for long-term use