One thing which will help is a photo or good drawing of the system's ducting.
There may be a spot where "all the air" passes through (say, after the heater core / evaporator box) which has a spot which would have enough "removable" plain duct so a filter and holder can be added.
One benefit you might not get is the CAF's ability to protect the AC evaporator given that you'll fit your CAF wherever you can.
If the cowl is the entry point for all incoming air, perhaps shoe-horning something like Volvo had in 850 series vehicles would work. Volvo's air intake was in the cowl on the right (passenger side) under the cowl. It was an open hole the size of a paperback book. Volvo had a frame which fit into the hole and accepted a rectangular filter.
Good luck with your project.
FUNNY: My friend got a used 1997 Ford F-150 and I found a genuine Ford kit to install a cabin air filter. The bottom of the glove box had a recess you cut away to gain access to a run of duct.
EDIT: I just noticed that the OP's post is 20 months old. ....wonder how he made out?