The grain of the diagonals should go from corner to corner. If all he did was make cut-outs in the sides, then the grain goes front to back. That makes for weak diagonals. If he were to glue some sticks behind the diagonals it would strengthen it tremendously but also put much of the weight back.

In any case, the plane is probably going to be strong enough as it is and I would just leave it alone. I was just making a point about trusses and what makes them strong.