Hurricanes Katrina and Rita Flood Insurance Buy-In Act of 2005 - Establishes a temporary flood insurance buy-in program. Instructs the Director of the Federal Emergency Management Agency to make flood insurance coverage available under such program for eligible structures (but not their contents) for flooding resulting from Hurricane Katrina or Hurricane Rita. Limits eligibility to structures located in an area not subject to the mandatory purchase requirements of the national flood insurance program, and which were not covered by such insurance at the time of the hurricanes. Prescribes a formula for determination of premiums, which may be paid by deduction from the amount of any claims payable under coverage by the buy-in program.