Hello Jim, everyone asks me about that. The layouts are only suggestions.
I got the 1x6 size from a website about benchwork construction, the author used 1x6 as his frame, said it gave him enough room to attach the panels where ever he needed them. I guess 1x3 would work as well.
As to the depth, it is set up for a dogbone mainline, 18" is the minimum radius, so I figured I needed the 42" depth for clearance. The 30" rear depth is to allow 24" for the module and a 6" rear gap for return line.
At 50, I am still active and bending is not a problem. But I guess you are correct, who knows what 4 years will bring.