I'm sure that some people here who shoot video with photo cameras have used one of the Hoodman Loupes for 3-inch viewscreens. Has your experience with it been good and do you have any clever solutions for solidly attaching it to a camera for continuous use as a viewfinder? You can buy an elastic strap to do this, but it seems to cost too much and many users say it doesn't keep it in position very well. It's available at the best price I've found at B&H Photo-Video: Hoodman HoodLoupe Professional LCD Screen Loupe H-LPP3 B&H Photo

I have one and have used it a number of times on my Sony Nex. It works quite well for what it is, and the straps that they use are quite effective, although I think you could likely do just as well with a couple of elastic bands.
it's effective in bright sunlight especially, however it adds bulk to your system if you're using a DSLR, and it's awkward at times, especially if you need to go back and forth from viewfinder/loupe to LCD.

I have one and have used it a number of times on my Sony Nex. It works quite well for what it is, and the straps that they use are quite effective, although I think you could likely do just as well with a couple of elastic bands. It's effective in bright sunlight especially, however it adds bulk to your system if you're using a DSLR, and it's awkward at times, especially if you need to go back and forth from viewfinder/loupe to LCD.

Thanks very much, Wayne. I've figured out a way to attach it with elastic cord I can buy at a fabric store for just a few bucks. Black rubber surgical tubing could also be used. I would use four small pegs of wood attached to the edges of the viewscreen with removable shoe goo (plumber's goop), to keep it from sliding out of place. I'd paint them black and they wouldn't be very visible.

Added note: I've been thinking that putting pegs on the viewscreen wouldn't be necessary. Just a few strips of black duct tape around the edges would keep the viewer from sliding. It might even be possible to use only duct tape around the edges to keep it attached and no straps would be needed. I'd try using Gorilla Tape, which is twice as thick and strong as the regular type and it could be quickly removed when I wanted to put away the camera for the day.

Wayne, could you give me the exact dimensions of the outside margins of the large end of the hood, where it contacts the viewscreen?

you bet.
outside measurements are
2 7/8 in. wide, 2 51/4 in tall ( measured in the orientation you would have it mounted)

I should have finished what I was going to say before hitting the "send" key.... I think fabric store elastic cord and a couple of small cord locks would be the best solution. Thats effectively what the hoodman system is, replacing the elastic cord with stretch rubber. the whole thing is secured by a loop of rubber around the eyepeice, which could be made from an o-ring 1/34/ in OD

I went to a large, warehouse-type fabric supply company and found some very good round, fabric-covered elastic cord. The 1/8-inch size was 77 cents a yard. I got two yards of that and also of the 3/16-inch size. With some split rings from a fishing shop, I will make two elastic loops for each side of a camera. I'll use some 3/32-inch braided nylon cord to tie a loop around the eyepiece and thread it through a split ring for each side, to anchor the straps. I'll spray paint this cord black before using it. Total cost, about $1.50 for a four-strap setup for a Hoodman Loupe.

The best solution costs a bit:

Cinema Kit Pro-Hoodman Corporation (http://www.hoodmanusa.com/products.asp?dept=1066)

I got the loupe first (before they came out with the magnifying eyepiece and crane), held it on the T1i (first HD video DSLR I used) with long rubber bands, a "so-so" solution.

Then I replaced that with the CAVISION system and used the Hoodloupe on a lanyard (kept it in the camera bag for when I just had the camera along.

The CAVISION system moved over to the 7D when I got that and when I sold the T1i and replaced it with the T2i I ordered the 3X eyepiece for the Hoodloupe and the Hoodcrane to use with the T2i.

Worth every penny of the price. Slips into the shoe on top of the camera and locks down nice. If I need access to the viewfinder eyepiece on the camera there is a "swing away" provision but for me it takes 10 seconds to loosen the knurled nut on the shoe mount and slide it off. 10 seconds to put it back.

Moving it to the 7D it takes 30 seconds or less to re-adjust and lock positioning of the loupe.

The 3x eyepiece is just about right for DSLRs with modern fine screens and awkward though it may look the "kit" is for me the most convenient, fast attaching, and smooth working LCD viewfinder setup on the market. I still use the CAVISION setup on the 7D but it takes me around 5 minutes to mount it in place.

I tried it on my new 60D! Slight lateral and inward adjustments (about 10 seconds), you have 3 adjustments that are a breeze to do.