Guys,
I have a ARF 90 size airplane, on which it seems that there is no right thrust, and it needs some.
What would be the best way to add right thrust ?
Perhaps make a plywood tapered plate and put that between the mount and firewall ?
Look forward to your suggestions.
-Gaurav

How about doing a mix to add right right rudder with the throttle ?

Adding a rudder trim or rudder mix is like cheating   

Using thin sheets of ply or really wide washers (so that the load is spread over a wide area) may be better.

You can get exact angle aluminum thrust wedges machined from a local machine shop, buy off the shelf thrust plates from tower, or simply add washers to the mount/below engine(if mounted 90 degree on side). 

From my experience, most - if not all - of the ARFs I have put together have the down/side thrust inbuilt. The firewall is angled and is sometimes not easily noticed.

I have not had to modify the thrust angle in any of these, so far. Presumably, they all had it or it is not quite as important as it is made out to be.