Is it possible to plant hedging along any of the borders? Stick in some native hedging (blackthorn, buckthorn, hawthorn plus some non-thorny stuff) whips in this autumn and you’ll be intruder proof in two years (so one for the long term, but still!).
Pros: looks great, good for wildlife, cheap
Cons: the thornier the hedge the more of a pain cutting it is, have to wait a few years for full effectiveness