Removing floor boards to hide a safe is a fantastic idea. I have a "In between" the studs wall safe that I hide in a certain spot in my house. Its low security but conceals VERY WELL. Its only 18" tall by 10" wide and 4" inches deep so can fit in a rather large back pack and you can obtain one from the local shopping store. There can also be small "gun safes" that can hold certain items and can be easily hide under the floors. Keep in mind that most floor are only between 6 to 8" tall by 16" wide so a big gun safe is not going to fit.