I get these caps made local for 55 bucks and charge 120 installed if I can walk up to it , I've never had a problem with it but there could be a better

I do the same thing,and sometimes paint them to match the roof, you can't see them from the ground. That one looks to be well made. We bought a 36" brake and mounted it in the shop so we can now make our own,and we keep a couple of the common sizes in stock.

Ya he keeps a doz made ready for me and one other guy , for what that wire cost does it save money to make your Owen

For those who remember Kirk Lapierre he did a youtube video on this.
http://kirklapierre.com/videos/how-to-make-a-ventilator-cover-kirk-lapierre-video/

I do them from the inside and have never had a complaint. Everyone says squirrels will nest on the wire but I think the fact that raccoons can get in those nests, prevents it. The raccoons can also reach in and feel my wire and then go to the next house.

Paul, I've seen three different burnt out fan motors from exactly that method. Fortunately they were homeowner or handyman done and not by me.

HY-C makes a good power roof vent guard. Doug fields also has some nice vent covers and other products.

http://fieldsdevsupply.com/

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David, what would make the motors burn out if the screen is on the inside vs. outside?

Inside is nice. But the photos look good too!

David, why the burn-out?

WWhen a squirrel makes a nest on the mesh, the fan can get stuck causing the moter to overheat and burn out. Is what David is talking about I believe.

Yep, a fan full of leaves can't turn. I was honestly surprised not to find fire damage.

NWCOA did research on this I herd claiming we were burning down houses , NWCOA said to stop putting on the inside because squirrel and birds were nesting on it , witch caused the motor to burn , attic is not a good place for fire , I know I see we're past company's were doing this there's straw and strings hanging down a few feet . I thought everybody had stopped . It doesn't seem to hold up very good ether . I get my insurance from NWCOA so I should follow what there trying to tell me , I fig someone was taking the hood off and had some cool way of rigging something then putting it back on , well any way if your still doing please stop

Around here, I have found that most of the fans don’t work in the first place. Of those that do work, if screen were placed on the inside, any leaves above them would make them useless anyway. I usually replaced the original screen with very heavy-duty mesh; invisible and the fan still serves its purpose.

How do you secure the top of the heavy duty mess

Ron, that would be ideal. If you could explain how it would be greatly appreciated.
I see that older models would come with either quarter inch hardware cloth or similar sized expanded metal lath, usually firmly secured. Anything new today has at best plastic screening and fiberglass screen seems to be the norm.

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Around here they have a temp gauge on them , people tell me all the time they don't work , what they don't know is there temp.activated , I'm sure you guys know this because the attic fans are more common in the south ,they must just be old and worn out

Plastic topped attic fan gets a HY-C RVG3030 since an excluded critter WILL destroy the top to get back in if I just do a screen replacement. Metal top gets either the HY-C rig or a 1/3rd inch stainless mesh refit that mates closely with the underside of the top (customer's choice). I've seen too many nests in fans either with or without somebody screening from below. I don't want to be the one they point the finger at if the place burns down, so I never screen from below on a motorized vent. Static vents, that's a different story.

Take the cover off, use quality 1/2 by 1 " hardware cloth and make a circle around the fan fixture, cut down to the top of the brackets and fold down over the top of fan, cut off anything in the way, use self tapping screws and secure the top back over the fan fixture. Never took pictures, sorry.
Cover over top has many advantages and can be removed and work performed on fan.
A few months ago I revisited a customer on an unrelated issue and notice a fan cover resting on the chimney and asked the customer what was going on and how did the cover get there. Turned out that They had a new roof and the roofers put it there and forgot to reinstall.