Heat is your number two enemy, water is your number one enemy. With those factors in mind:
- Plywood does not conduct heat.
- if you use a bag it must be big enough to contain plenty of air
- bags need to be vented without letting water in
- commercial plastic controller boxes get hot inside
If you need to cool the battery, but I must admit that in almost 10 years of e-biking I have never noticed much heat from any of the Li-ion batteries I have owned, but naturally, if your battery does get hot and that it is a concern, small holes, low down and maybe a tiny fan (CPU Type), should sort that out easily. See here:-
http://www.ebikeschool.com/extend-electric-bicycles-range-keeping-battery-cool/
Furthermore, if some battery types get warm/hot (not good for any battery type as the answer above suggests), then possibly a small, cheap thermometer might be considered to be installed.
Remember, heating up dramatically, is a sign of "INEFFICIENCY" in any battery, and really should not happen.
I am also careful in summer, to avoid leaving my bike in the sun, but even last year, with a white bike (black may heat up quite dramatically!), I could easily rest my hand on any part of the bike, especially the battery compartment. It was basically cool, no sensation of heat, even after a 20 mile ride. But I do have high quality cells.
But your comments need wise consideration and for each of us to be careful and due concern must be given.
I found a good and accurate website talking really good sense about Li-ion batteries, the link is below, but it must be stated here that a Li-ion batteries, when correctly sized and used, do not get dangerously hot on their own. The website does not even mention the problems, which fits in with my knowledge.
Physical damage, external heating effects like the Sun or a bad storage place, can cause a battery to overheat and maybe catch fire. Also, the quality of the manufacturing plays a huge role, as there is a lot of "cheap c**p" around!
Also, some re-sellers make vast claims just to up their prices! Don't get caught!
I myself prefer a good name of battery, with maybe lower "claimed "performance, for a reasonable price.....
I have never "lost" a battery myself, my last bike was sold with the original battery still working, at 7 years old! As good as still full capacity, but I do know how to "treat" batteries as well.
But I have also seen the fires caused by some biker's batteries on Youtube videos, UGH!!
https://electronics.howstuffworks.com/everyday-tech/lithium-ion-battery2.htm
I feel that it will be very easy to build a good quality box, with all the features needed, for very little money! Only some common senseis needed anda small amount of DIY woodworking knowledge.
By the way, I possess a MIG welder, and I could build one in metal quite easily if I so wished, but wood appeals to me personally far more......
And if you have no welder, and/or no knowledge of welding, the wooden one must appeal!!
Regards to all for a great 2019!
Andy