This is a rig so that you can top feed filament into a core one. I made this for TPU printing but it can be for any. It was designed to use 32mm plumbing pipe for the up-right tubes but I have included an STL for these. It is fitted with holes in the base for 5x5mm magnets so it can fit on the top of the core one and is held togethr with m3x10 screws. 

Instructions

First, get all of the respective parts.  you will need to print:

4x base plates

2x TPU apex assembly

1x TPU top tube male

1x TPU top tube female

4x TPU side tube (full) or 4x TPU side tube half (flat) and 4x TPU side tube half (angle)

2x TPU corner (Fl and Br)

2x TPU corner (Fr and Bl)

You will need to buy/find:

8x M3x10 screws

8x 5x5 magnets

Then, remove the top perspex pannel on the core one.

Screw the base and corner plates together and instert the magnets into the base plates. Make sure the the corners are correct so the Front left and back left are facing eachother and the Front right and back right are aswell. The screws should screw stright into the material. The magnets should friction fit into the holes on the base but if they done use some glue to keep them in.

Then either put the two halfs for the up-right tubes together. they should friction fit if not secure with glue. If you are using plumbing pipe cut some 32mm pipe so one end is at a 60 degree angle and the the longest edge is 285mm long. 

Once this is done everyting should just push fit together. See photo for the final construction.

All of the printed parts shoudlbe able to be printed on a core one appart from the full side tube but thast why I split it in two.

If there are any questions please feel free to ask.