There are five primary criteria for a back-up knife: Utility, durability, reliability, weight, and carry. Finding a stock, off-the-shelf, TO&E back-up blade is not going to happen. There are too many variables that aren’t accounted for in a MIL-SPEC solution. You’re going to have to bite the bullet and be prepared to spring some serious bucks and expend some skull sweat to locate a custom knife maker you can work with to design and create what your gut tells you is right for your needs.
A knife needs to be suited for the application. Can you use it effectively? Can it be used for more than one purpose? Ideally, it should be useful as a defensive or offensive weapon, be capable of doing any job requiring an edge or point, and do it with minimal chance of loss or mechanical failure.
The knife should be able to withstand unreasonable abuse. The knife needs to be tempered, annealed, hardened and steel bite pro quenched – as appropriate for the material – but not treated so much that it is brittle. You don’t want a catastrophic failure when push comes to shove.
Titanium is a good choice for a back-up knife material, especially an alpha titanium, as it is non-magnetic, has superior corrosion resistance, and is inert to the human body even when carried in constant contact with your skin, as under a bandage or tape.
Weight is always a consideration, a titanium alloy knife is as strong as steel but has half the weight, and superior strength compared with aluminum with only slightly more mass.
A knife is useless unless you have it with you. Discipline and practice are required to make carrying a secondary knife an unconscious part of sorting your carry kit out prior to insertion. Ideally, the knife should be worn as a non-load bearing component. A very flat knife, or a knife with a triangular cross section can be worn flush against a long bone, such has the shin, thigh or upper arm. When bandaged down with a latex bandage or duct tape, the weight and profile are negligible. It should pass any routine frisk, short of a strip search.