How to stretch out plastic picatinny multi-rail conversion component for airsoft pistol?

I recently bought a Palco M1911 style airsoft pistol (from the "Rail Gun" series) that has a short under slung picatinny rail. I had some Amazon money so I sprung for a few components, including a laser sight and a small cheapo red dot scope. The scope needed a rail on top to mount to, so I also bought a 4-rail conversion piece that slides on over the muzzle, attaches to the existing rail on the bottom, drops that one down 1/4", and provides 2 optional side rails and a top rail for optics.

But when I got it, it was too small, and when I could even get it on, it got stuck and I had to rip it off. It was advertised to fit the 1911 but the bag it came in says Walther P99.

Long story short, I spent the money and I have the bit; does anyone know how I could stretch out the plastic to make it a touch taller, and wider on the sides?

(Picture: Top: Multi-Rail converter, Mid: Palco M1911 Rail Gun airsoft pistol, Bottom: Bag from multi-rail converter)