If you are going to adjust the belt by checking the deflection, incorporate that into the design. A hole in the top of the shield, with a flexible covering, cough duct tape cough. 5 cent fix.
Seems like you need an adustable double sheave idler pulley mounted to adjust perpendicular to the belt length or slots for the engine to move, no?
I had doubts about building your own. I am impressed.