Recommend using Raft for safe and easy removal of the print from the build plate. This should ensure the print joints are not damaged during removal.
Do not use any support as that may cause the tentacle joints to be made solid.
Be patient and take care with raft removal so (again) no joints are damaged. With a well set up machine the raft should remove cleanly and joint move freely immediately after Raft removal.
3Shells and 20 - 25% infill is suggested to ensure the joints are strong and will be durable in use.
Attach magnets to the underside...
To complete your Octo Magnet you will need to get some reasonably strong magnets as follows:
9 x up to 12mm diameter x approx. 3mm high
1 x up to 20mm diameter x approx. 3mm high
Note: All magnets should be the same height. Suitable magnets should be available from hardware / craft / electronics stores.
Glue magnets into the recesses on the underside with some strong glue. When the glue has dried your Octo Magnet will be ready to use. You can hold up to nine notes/photos etc.
Make sure you find reasonably strong magnets so they will secure thick paper or light card.