We did an evaporator on a Dodge ram (gen III, i think IIRC) and while it didn't have the filter, it did have a molded spot where it would have gone.
My friend said that Dodge dealership used to offer a kit but it was NLA.

I researched it and the only difference in the kit VS just the replacement cover (replaced a broken or lost cabin air filter cover) was that the kit included a filter. I bought the cover for that model and the guys cut open the hatch spot on the air box. All the track and what not was molded into airbox.

I can't imagine why they didn't just put a cover on them all and choose to run a filter or not...

HomeDepot sells a 'no rinse' aerosol evaporator cleaner for home HVAC. You spray it on, let it foam for a while, then run the A/C and the condensation washes the evaporator clean (thus, no rinse) I am going to do that on my cars. Remove the blower (or cabin air filter) and spray it in onto the evaporator. Can even flush with a bug sprayer (new one) filled with water.

BG Products sells a Frigi Fresh kit that has a foaming cleaner like that and a duct sanitizer / deodorizer. Available on Amazon.
Toyota also has a two part aerosol kit like this.

 

Yes, I was hoping there was a slot for a cabin filter in the evaporator housing, but unfortunately they made no such provision; it's too bad they didn't do this.

The first pic below is the cabin filter that was available as an add-on for these vehicles. They're hard to find and pricey if you can get one:

Original cabin filter (behind cowl) available as accessory;

Luckily on the 3rd gen NS vans, access to inspect and clean the evaporator core is pretty easy via the temperature probe cover, which pops off and on easily. This enables cleaning the core with the aerosol cleaner you mentioned. It also gives me the idea of using a regular home a/c foam filter if I can figure a way to attach it.

The evaporator probe cover behind the glove box gives access to the front of the evaporator core. The temp probe cover is just held in by a rubber gasket and can be popped in and out easily. Access only requires removal of glove box door, which is easy.

Inside the evaporator probe door there's a clear view of the evaporator core. This is the new core I installed.

I may try to insert one of these portable home a/c foam filters in front of the evaporator. I may need to hang the foam by some plastic pins inserted into the evaporator core. I haven't thought this out enough yet. These foam filters come in different thicknesses so it's probably best to buy one of the thicker ones rather than a flimsy thin one. Also, it's probably necessary to put some kind of spacer between the filter and the evaporator core, as exists on portable home a/c's, like some kind of plastic grid spacer. This will prevent the filter from retaining water on the front of the fins, which might accelerate corrosion.