I've had 240's for 25 years. When they went to the new F.I. systems that included an AMM (in contrast to, e.g., the '80 245), it got harder to access the air filter box.

There are only two clips, not four as you expected. The two top clips are accompanied by two "pegs" that fit into slots at the bottom of the box (where you can't see them). Always be sure those pegs are properly inserted in the slots when you close the air filter box, or you will leave a gaping hole that will render your filter useless.

The best way, as Matt first suggested, is to loosen the bolt on the AMM, then loosen the rings on the air duct elbow, and slip the elbow off both of its ends and put it aside. Then you have easy access to the filter box, and can be sure you're installing the filter and closing the cover properly. Then reverse the process. Although this takes a little longer, I like being able to see that the whole air filter box cover is properly in place -- it's worth the extra few minutes.