Hil Wal, on the small press the plate slides snugly between the side plates which guide it. On the big one there are stepped rollers rather like the ones you show in your second shot but the platen rests on the shoulder of the step. Trying to machine a groove in a large steel plate is probably best avoided. I don't see any reason why you wouldn't go for the first approach TBH, quick and easy. Most of the plate support comes from the bottom roller, it rests on one of the side plate spacers, which one depends on where it is.