I was in Bunnings the other day and they had a whole stack of WeatherTex panels that are 145mm wide.
WeatherTex is the stuff that I see they are using for bottom boards (although they use the boards, not the panels), as they stand up to the elements and are harmless to bees (they are chipped hardwood mixed with parrafin wax).
Anyway, seeing as they are the perfect height for Ideal supers, I thought I'd experiment with making one.
I made them using rebate joins. After cutting the sides to length I glued two pieces together to double the thickness
(they are only 9.5mm thick). Both the front and back are also made of two pieces to increase the thickness, however
I ripped ~10mm from width of the inner pieces, as well as cut them ~40mm shorter. This is how you end up with the
rebate join and the rebate for the frames to sit on.
For $1.80 a meter (they come in 1.8m lengths), they cost about ~$5 for an Ideal super, that is weatherproof and safe for bees. It's going to be approx double that to buy the hoop pine needed for a single box, and more than triple that if you buy one with the joins already cut. The main downside is that it is heavier (being hardwood). I haven't weighed it, but picking up an empty one of these compared to a pine one, you can notice it. When they are both full of honey, perhaps you won't be able to?