Do not worry about mice - a single queen can chase it away like in this (german) documentary from minute 8 - 9:

I have built a bumblebee hotel a few years ago following these instructions:

http://www.hummelfreund.com/basteltipps/bauanleitu... I can only recommend two things:

1. Never place the nest in direct sunlight - it gets too hot inside and above 41°C the bumblebees die. Always have something around it

2. Place it outside at the right time (quite early in the year) when the queens are looking for a colony.