Something like this is probably what you need:

Volkswagen Golf IV 1.8T > Tools > Suspension > ES#2219682 Strut Nut Wrench - ECS010

There's an ECS group buy going on atm in the MK4 forums, so depending on how urgently you need the tool it might be a good chance for you to pick it up along with a few other things.

There's also a way to do it using a spark plug socket I think. Marty talks about it in the latest Mighty Car Mods "Budget Street Cred" episode where they do up that MK3 Golf. Fast forward to the bit where they talk about the JOM coilovers for the car...or watch the whole thing if you haven't seen it yet, it's a great episode !

Mighty Car Mods - Budget Street Cred (Season Finale) - YouTube