Storing squash

Wood probably if you have a choice. I keep mine on my window ledges in the conservatory and they're plastic coated

Keep a check on them and turn them regularly. You need to check each end for signs of mould. If you do see a bit of mould then you need to eat that one 

I prefer direct sunlight as mould does not like it and the squashes harden and improve in flavour faster!
As for rack material i think non porous is better than wood , i use ex-oven steel racks which work well.