boat rack

G'day old baz'
The way I understand your dilema is that you have a roofrack on your disco but it is to narrow for your tinny.If I have got it right then you could simply bolt 2 or 3 wider beams on the top of your existing rack [you could even put rollers on the beams if you so desire]. With doing it this way you can then just unbolt the extensions when you are not taking the boat with you.All the best!!!

Troopytrek!!!!