I made a gravity filter a while ago and had a similar problem with the tube sizing. I went to the local hardware store and bought some rubber tubing that fit over the filter's spout, but inside (with a bit of work) the hose I was using. After fitting it all together, I wrapped the tubing/spout interface with waxed nylon string tightly, like fishing poles guides are attached to the rod.
The location of the filter shouldn't make much difference in performance. For a platypus system, you will want the filter inside your pack to keep it from bouncing against your chest while hiking; reducing the chance of it coming loose. I've seen the gravity filters with the filter at the water reservoir or the filter at the end. Both seem to work the same.