I used the 3/8" sleeves for my fittings, I tried the 1/2 " but it would get baggie on the inside of the radius when I made the arch. Not sure how well the glue stick will come out of the fitting once the epoxy sets. for the internal mold the method I used was with a length of stretchy rubber tube. It had a wire inside of it to hold the shape and diameter. Once the epoxy sets you will simply pull out the wire and then stretch the tube by pulling on both ends. That will release the tube from the epoxy and it will pull right out of the sleeve. The method I used looked much like what was shown in the prior post from Rick D.
Enjoy your build,