I got a harness from local clothes shop, can't remember the name of it but a set of reigns type harness, kiddi something was the name, and it cost about £5 you can get them in chemists etc... I used it on stokke tripp trapp.
we still have that high chair and plan to keep it, I would have bought the ikea one if I really wanted something that was cheap but good.
I know you want a tray, but even with a tray I have noticed food all over the place with friend's highchairs...
the stokke is good because it isn't just for eating food, it is great when they are oler and you can have them sitting at the table drawing, playing with playdough, but they are still contained.
I think it can be a trip trap like the name suggests!