The only way you are going to properly seal that cap is to remove the spring mechanism completely and use a thick rubber sheet, cut a circle and place under the cap as a seal. Also use a thin layer of subarubond or hondabond on the cap. The cap should fit extremely tight on to the neck ( I used pliers to tighten it down. and then zip tied it off) in order for it not to leak. I suggest just getting a WRX radiator and save yourself the headache. I've been there and there is no good way of sealing that cap off.
another option is- You could tehnically have it plastic welded closed.