We have a Carrier gas furnace installed about a year ago. I'd like to
replace the filter but need some advice. The manual that came with it is
very generic and not very helpful.

There's an existing filter in a "box" that sets on top of the furnace. The
air return attaches to the top of the box and the air passes down through
it. The filter sets over a rectangular opening in the bottom of the
box, supported on the rim of the opening. The opening has no cross braces
or supports, just a large rectangular hole with a narrow edge that
supports the filter. It looks like it would just take a standard(?) 16x25
filter.

My question is will a standard filter have sufficient stiffness not to
buckle and get sucked into the furnace? Or will I have to put in some kind
of sheet metal grill or other supports?

The existing filter is kind of coarse mesh stuff and seems like it won't
trap fine particles, which is why I want to replace it. It has a couple of
steel rods passing through it to give it some stiffness. But it's not in a
cardboard frame like the filters we used in our old furnace.

Thanks for your help.

-- Joel