Looking good! The only problem I can see your going to have is filling in the joints.
I don't know if its any help now but this is how I do it....

Holding the knife in place, turn the plastic pipe under the blade and cut a groove into the pipe.As opposed to cutting all the way through the pipe just snap it off. This will give a cleaner end....

Cut two rough squares out of thin plastic card bigger than the diameter of the pipe.

Glue both squares to either end of the pipe and leave it to set.

Once its glued, rough cut away the spare plastic card.

After thats done, use a modeling file to smooth away any jaggy corners and hey presto!