Hi King, and Harlan too.
Firstly, I am always open to suggestions so always feel free to comment.
I'll try to address some of your questions, and also what I have in mind for Mod 2 on this.
I know the EPA stations use HEPA filters, I believe the rational is if they start seeing high counts they can send the filter into the lab for spectral analysis to see exactly what type of nucleotide they are dealing with, Cs, Pu, etc. but I am hardly an expert on the subject.

I know the GM tube is located on the bottom of the unit as I popped the cover on it for a look see. Nice electronics on the GMC, but the Chinese GM tube they use leaves a lot to be desired. Alright for the money, but it can't begin to compare to my Ludlum survey meter with it's pancake tube.

I do have two professional grade Geiger counters on hand to verify the readings of the GMC if things get wonky.

Here's a more detailed explanation of how this is set up, just to clarify things, and you can refer to the not so good picture at the bottom of this post.

The intake port is on the left side of the box (2 inch pvc elbow) butted against it is the bottom of the GMC-320. The fan on the right side is blowing outward, so when the cover and gasket are on, the left port is the intake and the right port is the exhaust. The fan is 60 MM in diameter and is rated at 25 CFM (which is pretty phenomenal for its size). As the box is physically not that big, I believe that when it is closed up, the airflow should be more than adequate. I could have used a bigger enclosure and fan, but everything you see in this setup I already had on hand, and did not have to buy anything, (I hate the "B" word). I do have some course filtering material crammed in the intake port, which you can't see in the picture, mainly to keep sand out.
So, here's what I am thinking to improve this:

The GMC is mounted using adhesive backed Velcro tape, so I have an inch or two wiggle room. I am going to try and find a air filter such as used on a lawn mower engine, or such, the correct size, and mount it on the inside covering the intake port, then butt the bottom of the GMC right up against it. Guess I should have thought of the earlier.

Let me know what you think of this idea, and if it would be a viable improvement....

I would highly suggest a HEPA filter only. You could cut up and use the Holmes brand filter like I use easily found at WalMart.

I own 3 pancake geiger counters and while they are great units, they would not be good to use like we are doing outside 24/7 and with fans. The GMC-xxx GM tubes win hands down for durability & cost when compared to the pancake GM type.

The suction alone from the fan could damage a pancake GM tube. If the suction don't, likely the moisture would. If neither of those damaged it, there is always just risk of failure and with a 90-day warranty on the pancake tubes, that is a costly replacement.

Ok, sounds good to me. What department in WalMart do they have these filters?

In Arkansas, the filters are next to the water filtering systems.

Thanks Harlan.

Sorry for my late reply, thanks hey/Harlan

@EB0FCB7A - You might be fine using a furnace filter that is or close to HEPA standards. This would likely be the most cost effective choice and yo could cut out what you need.

Hi King
There's an idea. One other thing that occurred to me would be to try a HEPA rated vacuum cleaner bag. That seems to be the best value or the money, especially since I am probably going to have to change it every two weeks here in the desert. Of course I don't know how much it would cut down on the airflow. Know anyone who has tried a HEPA vacuum bag?

I never thought of vacuum bags, thanks for the tip! I will confess, I do not change my filter as regular as I should.

Ok, here is Mod 2.
This is a HEPA rated vacuum bag on intake butted against the bottom of the GMC.
The HEPA bag is a heck of a lot thicker than normal bags and I could barely blow through it using lung power, but surprisingly it did not cut down the air flow as much as I thought it would once the enclosure was sealed back up.
I guess I will try this for a couple of weeks and see how clogged up it gets. Once again I had to clear the data history, so I am starting from scratch...

Looks really good. Since your unit is sealed so well from water and sunlight, you might consider removing the GMC geiger counter cover. This will allow the geiger tube better detection off the filter.