Bird Box Construction

Hi Observers,

   I posted recently about starting a bird box scheme from scratch. Thanks to Seaman and Mike B progress since then has been rapid. One goal is to construct the boxes, for blue and great tits, from design to building. I have a volunteer who has the skills to make these and to start I need a link or somebody with a good knowledge or experience of the construction.

A) They need to be suitable for the two birds mentioned. They are the only ones I know in this area (East Anglia) that will use boxes.

B) Does anyone have a design? With dimensions and suggested material?

C) Drawback. There are, not many, grey squirrels who could attack the fledglings. In fact they will chew away to gain access to the boxes.

Are there any suggestions please? Ideas? Squirrel proof design? How do I attach the boxes without using nails, bolts or anything that would go into the bark of the tree as this is forbidden. Types of straps or loops?

Can you help me please?

Kind regards,