Make the tube yourself if you can. Brass or Alum or even plastic. I've seen guys effectively use drinking straws even. Nine mm depth from LED tip to end of the tube, and as narrow as possible for the Emitter. DO PAINT THE INSIDE WITH FLAT PAINT. Or else you get "IR Splash" tot eh left and right (really bad on metal emitter tubes) which results in many "Friendly Fire" issues. Don't paint over the emitter either. Glue it in place with rubber cement (easy to remove and reinstall). Some of the nicer tubes you can buy have retaining clips and or screws. On a Stug put it where HL Had their IR emitter. Blow the IR tube out with air every now and then with canned air to keep out dust, dirt and FOD.