Elmer's glue in the US!It's cheap,safe,and there really isn't anything easier to find
It is also available in the UK apparently,but regardless of that it is the only glue I've seen to have the courage to essentially state in a fairly public manor that they don't use animal parts which is a bonus.
You can just make levels out of wooden dowels and basswood sheets or popsicle sticks,along with Elmer's glue.No need for questionable(and more than likely unsafe)metal items or anything expensive,as you could certainly make loads of these for very little money!