Hilbert FIR Filter for Q - Matching Filter for I
Clay Turner has an interesting couple of papers.

what you want to do is compute (using MATLAB firpm() or firls()) two filters that shift the phase, one at -45° and the other at +45° relative to some linear phase angle that corresponds to a constant delay.  you design one and then flip the coefficients around for the other.  they will both have the same magnitude frequency response and be 90° out of phase of each other for every frequency.