Nesting box plate...

Hi all,

I put a bird box with camera up at the end of Feb which immediately (less than 24 hours) caught the attention of a pair of blue tits... 

They have been (slowly) bringing moss and grass in for the past 2 weeks now, and are still at it...

They have also been chomping at the hole which started at 32mm but is now somewhat larger.

I have since read that a 25mm hole is best for Blue tits to protect against predators and nest raiders, so I have purchased a metal plate...

However, I am now concerned that retro fitting the metal plate may disturb them?

The box is under beneath a first floor soffit so the only predators/raiders would be other birds.. 

Any thoughts/advice appreciated. 

Garry