I've made a few injectors, and I've found smaller diameter ones seem to inject easier. Maybe its just my workmanship since I don't make them everyday, but I've made a few. I never planned to make any injectors at all, but I had a similar situation. I made a mold for a customer and at the last minute he asked me where to get an injector big enough to fill it. Oops. LOL. I made three of them, and took all the best parts to make the one I sent him with his mold. Back then Bass Tackle didn't have one big enough to fill his mold. I haven't used every injector out there, but I have used a bunch of them and like I said in my other post I send my own mold customers to Bass Tackle to buy injectors.
Ha! The picture of the two I kept from that batch is still posted here. LOL.