Ceiling Register question

This is probably a stupid question, but I’ll ask because I’ve never done anything like this before. I just got my basement finished and the contractor never installed registers in the ceiling. Ductwork is all there and holes in the drywall are there and everything he just didn’t install the registers. So I figured no big deal. I picked up two registers and proceeded to screw them in. However it appears there are no holes in the duct for the screws to go into. They go through the drywall and then stop.

What’s the best way to solve this? Can I just take my drill and drill my own hole and proceed with the install?

 

It sounds like the register boxes are missing.

 

Can you snap a pic?

 

Really sorry for not responding earlier. I took a look and could definitely see the register boxes in both. So I think the screws in the new registers were not lining up. I ended up drilling some very small holes using the registers as guides. That gave the screws a place to go. Seems to have worked.