When you take carb off, check bottom of it with a ruler, if you see this kind of gap between ruler and the carb, then there is the problem:
Fixing that requires wet sandpaper, thick smooth piece of windowglass, a table and bit of time.
I find it is best to remove throttle butterfly shafts completely and carb top plate + other parts, so there is just carb base left and then sand that so that it is straight again, use plenty of water.
When putting carb back to motor, make sure you don't over tighten the bolts, those are only 2Nm or something as silly, you can almost tighten to that with your fingers, so best to use some locking stuff to keep bolts from opening.
Cornflake box is perhaps not petrol resistant? There is proper gasket material sold, it is not big of issue if you get thickest you can find, will cost next to nothing from place where car parts are sold.
After getting base straight, adjust floats, then turn all screws to base settings, all data can be found from technical section of this board, there are green book info from base settings.
Then it is required to adjust a carb again, but it should work a lot better after that.