I use Wickes Non Structural Hardwood Plywood:
https://www.wickes.co.uk/Wickes-Non-Structural-Hardwood-Plywood---18mm-x-1220mm-x-2440mm/p/110037
And use this for the side bars:
https://www.wickes.co.uk/Wickes-Whitewood-PSE-Timber---44-x-94-x-2400-mm-Pack-of-3/p/107048
A pack of 3 makes 45 bars.
I do not use marine ply as it does not stand up well to sun light exposure. There are a lot of misconceptions about marine ply
Water proofing can easily be achieved with the right coating.
The boxes are heavy compared to cedar, but can be produced very cheap.
I am only in my 2nd year of keeping bees so cannot vouch for how long they will last. However, I did put an unpainted brood box into service last year and it is still in good shape, no sign of delaminating – yet.
The coated ones from last year still look as good as the day they were made.
If you are interested in making hives but lack wood working experience, like I did, then I recommend starting out with some pine and ply.
2 tips if you plan to use ply, firstly do not get them to cut it to size in store. They will not cut it accurate enough. So, I get it cut into strips that are a bit wider than I need, then cut them to the exact size at home. And most 18mm ply is actually 17mm.
Now I am actually pretty good at making boxes I will be looking to upgrade to cedar if I can source some at a reasonable price.
I cannot offer much advice about looking after bees, I will leave that to the pros, but if there is anything you want to know about making ply hives, just ping me a PM.