Here's a 2*40MM mount for the rear of your Corsair 280X case. I had the two fans laying around, so that why I designed it for 2*40MM. Else, you're better of buying 2*60MM, and use a alternative mount for your case.
You should be able to use normal screws for the fan-mounting itself, but I had some spare Noctua mounthing. I'll recommend you to use 4* M2.5*6mm. Personally I didn't notice I grapped a larger one, so my installation are not the most pretty (so one knob broke off during installation).
The screw-mount could have used a bit more plastic, to make them stronger, but for a installation like this, I didn't spend time on making another draft version.
If you're looking to improve the air-flow even more in the Corsair 280X, I'll recommend you to check out the following designs from other creators:
The author marked this model as their own original creation.