As far as the silicone, you can buy "aquarium silicone" which is super-sticky so that it can adhere to the sides of the glass at most pet shops. Acrylic shelves can be purchased (and cut for free) at Lowes or similar. Don't forget to measure the inside dimensions of the tank, since it's really hard to cut it yourself once you get it home. If you look in the thread titled something like "your set-up" you can see a bunch of really interesting ideas on how to make tanks have more total space. How many mice are in the 3ft tank, and how tall is it?