I bought a BosScreen for mine and it worked very well. The BosScreen isn't a
true "brightening" screen but it has much the same effect because it spreads
the image evenly across the screen, eliminating the "hot spots" of a plain
ground glass. It's also easier to focus with than a Fresnel. If you end up
being the winning bidder for this camera I'd give the screen that's on it
now a try, it may work fine for you. I used three screens on my Deardorff,
the original non-Deardorff that came with the camera, a Deardorff screen I
found later, and the BosScreen. While the BosScreen was an improvement over
the other two, it wasn't like night and day better. I'm sure a Fresnel would
have brightened things more but I've always found Fresnels not so easy to
fine focus.

I have found that with long lenses, a Fresnel is not really necessary.
The trouble is with the short lenses (e.g., 90mm on 4x5). There,
everything is great if the lens is on-axis, but if you use any shifts or
rise or fall, the illumination tends to be worse than no Fresnel at all.
So I no longer use them. Ron Wisner has easily removeable Fresnel
screens for his camerae. Since I imagine they are moulded vinyl plastic,
you could probably get one of his and cut it to fit (if necessary). You
could probably call him and ask.

I bought a very thin, cut to size fresnel from Jeanie, J.M. Sharkey
<jmsharkey@hotmail.com>. Apparently a photographer and a single person
garage business. I'm happy with mine. Being a newbie, though, I don't know
how well it compares to a "real" fresnel. It brightens the GG pretty evenly,
I'm guessing about two stops. She (he?) claims the thickness is 0.016", but
I didn't mic it. On the two test shots I made specifically for focus shift,
it was certainly within my depth of focus, although I could have chosen a
more critical test subject. There was a single hair from my yellow lab stuck
to the side of a velvet lined case, running almost broadside to my line of
view. I just don't recall which end I focused on. Works fine enough in real
life, too.

Cost was $55 for 4x5, bought through eBay. I suspect direct private sales
are possible. I have no relationship with the seller except as a
non-disgruntled customer. If I weren't curious how a "real" factory fresnel
looks, I wouldn't hesitate in the future to buy a second one.

   I have Maxwell Screens (Maxwell Precision Optics) on both my Deardorff
4x5 Special and my 8x10 V8. My initial purchase was to aid in focus via his
Brilliant Fresnel and gridded focus screen on my 4x5. The screen was such an
improvement over the standard orig Deardorff GG on my 4x5 (Especially with
my 90 mm Fuji) that I had Bill make one for my 8x10 as well. Brightness is
not the only benefit as the image is rather much finer on his screen as
opposed to the orig acid etched GG. A real pleasure to use and absolutely no
"I wish I Had's!" on my part!!
Joseph Burke