Devcon is very good, it sets SOLID and is hard to distinguish from the aluminium around it when you run your finger over it. I have also used some cheap equivalent that i got from Halfords but that failed and never felt completely hard, atleast when compared to Devcon.

If all you are doing is smoothing airways then the cheap stuff is probably fine. If you are raising ports with a piece of aluminium and are using putty to fill around then get the good stuff. Also, never rely entirely on any putty to hold things in place, always find a way of bolting down the piece of metal (or whatever) you are using to raise the port. The putty can fail due to vibration. In fact, just get Devcon regardless of its price, coz it rocks.
_________________
After years of moaning about immigrants now i am one...