I started to make one out of ply in fact here is the picture of it below, but it turned out a lot more heavier than the tubes and will end up costing more once it is treated. To do this properly one needs to use marine ply which is even more expensive and then you need to treat it ever year or you may find the ply starts to peel and chip away.
Oh and another benefit of the tubes is that since I sail in freshwater I back the trailer in and out of the lake to launch and recover.
I remove stuff from the tubes first, they fill with water which stops the trailer from lifting / floating.
Once done pull away and the water drains straight out and they dry pretty quick too.
Not sure if I would attempt this with a wooden fabricated sail box.
Currently using it as a storage unit in my messy loft come office! (it's crap though! I am no carpenter that for sure, can't sing either!)
Here is half of it buried amongst a load of stuff! Really need to do a spring clean wife is moaning about it, lol!