Sealing up those aftermarket doors can make a huge difference! Most of the standard D-shaped or P-shaped automotive-grade weatherstripping seals you can get at an auto parts store or even on Amazon work pretty well. Make sure you clean the surface of the door and the frame really well with rubbing alcohol before you stick it on. That's key to making it last! Also take your time with the installation and you'll be surprised how much quieter and cleaner the cab is after.
I understand it's frustrating to buy an upgrade and still have to put in more work. The main issue is that aftermarket parts, including doors, are designed to fit a range of different models and aren't built to the exact same specifications as the parts that came from the factory. The factory parts have tighter tolerances, but even those aren't perfectly sealed because UTVs are meant to get dirty. So think of the weatherstripping as a way to customize the doors for your specific needs and the conditions you ride in. It's a small DIY fix that gets you a much better result than you would have with the stock setup, and it's a lot more affordable than a full-on custom-built cab!