I'm always interested in seatboxes,especially if they offer something which reduces weight and adds to their function. Only my opinion, but I've seen a lot lately that are impossibly heavy and require a full scaffolding with aluminium and plastic accoutrements to supply the pole rests etc
There's a lot to be said for the old style canal/compact seatboxes, not least that you can pick them up and walk off, and the detachable footplate, as opposed to cassette in subframe design is flexible ie you can leave it at home if you know you'll be on a flat bank. They're small car friendly too - this Boss model has a footprint that's only 21" x 14". I think Matchbox do similar models - the Boss is only a second-hand option - and people speak highly of them. Some of the older models have square sections legs, which are fine by me as |I have all the accessories I need, but could be a limitation when most these days are made to fit 25mm or 36mm round legs.
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At the risk of looking like I'm plugging this brand, I'll use the box I take mostly these days to illustrate something about attachments and all the add-ons you mention you might need for whichever you go for. Apart from being well-made etc etc, this box has an unusual feature - a 3cm square section base, with the ends open, front and back. You probably can't see, but I've taken the pic so I'll put it in
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I've found this allows me to swerve around a ton of clunky accessories by making use of the frame, and the fact that you can fit various square-section aluminium or steel tubes into it. You can add, to start with, wheels, handle to push, rod rests, pole spray bars....... and still have a compact, uncluttered set up. I notice you said you need stuff to fish long on the pole - I don't fish matches these days, so I generally choose places where I can fish 10m or less, so I can dispense with all the rollers and rests, (plug or cap the end section, and slide it on the grass/a holdall; trap it under my arm to feed) and a couple of top-kits rest on my holdall parallel with the water. The Octbox takes up a bit more room in the boot - footprint is 25" x 19"
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It's worth doing what you're doing, ie giving the seatbox a bit of thought, as there's a lot of scope to get a set up that suits you personally and avoids the millstone of a box you can't move and a set up that takes all day to assemble and disassemble.