I would recommend using a Holster designed for your particular firearm.
If you are bound and determined to make something "work" you can always put your gun in a plastic bag and shove it in the Holster for a day or so. I do not recommend this, and you do so at your own peril.
Select a gun, then a mode of carry and then a Belt Holster combo and be done with it is my advice. I'd have kept the Sig if it were me and sold a dog or mowed somebody's lawn to pay the car bill, but that's just me.
Sell the Holster on flea-bay and get one for your gun. You will ruin your Holster if you do the "plastic bag trick" and it will not work like a Holster designed for your firearm. You will also make it non-desirable to a Sig owner if you should try to sell it later. In short, you will create an exspensive piece of junk.