Gas traps were not manufactured after they were found to not work well. Any gas trap you see now was made that way by somebody with lots of time and money. In any case, assembling a rifle out of parts won't save you any money. It's no different than doing the same thing with a vehicle. You will learn a great deal about the Rifle though.
Using a European parts supplier opens a huge ugly can of worms. The EU requires enormous amounts of export paperwork and permits, usually by country. Some of 'em do not allow the exportation of parts for semi-autos at all.
If anybody ever tries to tell you they have an M1 Rifle with all matching numbers, smile politely and run. There's no such thing. Numbers on parts have nothing to do with the S/N. They're only drawing numbers.