After a few years using a stripped down version of an Octbox, I'm doing something very similar to your "top quality yesteryear standards".
I've just transferred all my bits and pieces into a mint Boss Compact box from the mid 90's. Empty, you can pick it up with one finger under the strap and the wood/fibreglass construction is svelte and pleasing, compared to the all-metal Octbox. I picked it up second-hand for less than the price of a space-station side tray. Its carrying capacity to weight ratio beats anything I can find on the market today.
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I've a bolt-on footrest if I need it, and I carry a small platform in the boot that sorts out any bank that leg adjustment and footplate can't make comfortable. Its construction appears lightweight, but quality and weight aren't the same. I'm still using an old version of the same box that's getting on for 30 years old, and I've failed to wear it out or break anything. Ok, I managed to wear the label out. I made a plain lid ie no drawers and use it when I'm fishing a simple method or have to rove about a bit. I've also been using it, having tried a couple of chairs, for barbel fishing as its much smaller footprint and long leg adjustment lets me fit it on small ledges on steep banks where chairs won't fit.
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The square legs are 18mm, once a popular size, so as you say there are lots of old attachments going cheap, and besides I've little interest in attaching things to boxes beyond a side tray of some sort. I'll even resort to radical options like sticking a rod rest in the ground or putting a bowl of groundbait on the floor.