Welcome to the forum.
Similar to what Kev says, I use a chair if I am going feeder fishing on the gravel pits, apart from that it's a seatbox,
I used to have a Shakespeare box many years ago and I was lucky enough to be friends with the "Octoplus" rep.
He gave me a set of the adjustable legs to try out.When they were about to go on general release, Octoplus were looking for feedback from anglers. They were ideal for uneven banks.
If you still have your box and wish to save money. Pick a set of these up off eBay. Attachments such as side trays are still available.
Upto to date boxes have drawers both front and side ,tray units. ,Some have base units.
If you go down that route, decide if to need a footplate, some are removeble on boxes,some are not, they add extra weight, Go for an H frame box, you can adjust the height without adding extra drawer units,
The best river box I have found is and AS1 , Strong ,sturdy ,virtual bomb proof, some say heavy, but I would match it against any modern box with a similar capacity. .again you would have to look on eBay.