It all started out when I couldn't find the safest method to l conceal my house key after locking my door on Shabbat. I'd hidden my key under the mat in front of my door, or buried it in the yard, but would always be worried that someone may have seen me do it. A Shabbat lock was not an option as I did not own the house I was living in at the time and installing one was not permitted. 

I came across a few belt options in stores and with that started looking into DIY belts to try and fit my needs. While they came closest in security, they didn’t seem halachically sound to my rabbannim and me. A lot of the belts on the market used the key as a secondary attachment (tafel) rather than utilizing its functionality. It essentially turned the belt into a larger key chain, thus making it asur. 

When the key itself is considered an item of clothing as a matter of function, (for example if the key is made out of silver or gold and can genuinely be used as a type of jewelry) it would not constitute a breach of the Laws of tiltul (carrying) on Shabbat as some sources indicate. However, most keys today are not made in a way to be warn as jewelry. As such, the sources below explicitly stress that one may only wear a non-jewelry type key when it is used as an intrinsically secondary (tafel) part of a garment. Our Shabbat Belt fulfills a requirement of necessity as the key itself is used to fasten the belt. 
I encourage you to investigate the following sources and speak with your Rav before making a decision.