Now that I've acquired a few "vintage" portrait lenses, and with the hopefully impending delivery of a Lomograflok Instax back, I've been thinking some ND filters might be useful.

The lenses are all f/4.5:
7 1/2" Ilex Paragon Anastigmat - 1/200 max shutter speed
9 1/2" Heliar - 1/100 max shutter speed
12" Wollensak Velostigmat Series II - a whopping 1/50 max shutter speed

I'm using these on a Toyo 45g

Haven't gotten outside with these lenses yet but I'm thinking with the limited shutter speed it will be difficult to get to the wider apertures in daylight even with ISO 100 film (I currently use FP-4+ or Portra 160). And then of course there's the ISO 800 rating of Instax.

I have a Lee Foundation style holder from Filter Dude for 4x4 filters which works on my more modern lenses, but these older lenses don't seem to have the standard filter threads to attach the adapter ring to. I guess just holding a 4x4 filter in front of the lens wouldn't be horrible (should I tape the edges for a little more protection?). Or maybe the Toyo compendium lens shade would be able to hold them? Any other ideas?

Any suggestions or experiences regarding density factor? I'd obviously try to avoid direct midday sunlight for aesthetic reasons, so more like open shade and golden hour. A set of .3, .6 and .9 seems like it would give a good amount of flexibility, but might get a bit expensive.

Which brings me to brands / lines. The price range seems pretty wide, but $180 give or take seems to be about average for 4x4" filters. Should I really be looking at about $500 for a good set of ND filters? Should I look at less expensive or one or two filters? Stick with Lee, Tiffen, Hitech? Glass vs resin? Recommendations are appreciated.