Wow, so simple, yet so obviously effective.
Ricohman. The item Art has shown is a great idea. Not sure how much it cost, but it seems pretty straightforwar d. The type I discussed are sold as universal type compressors (see updated image). While the one Art recommends looks like it hooks under the cam and pivots on top of the valve retainer I'd think it would work equally well on a standard rocker shaft too. In fact I've used a short, round bar about 2" long with a hole in the center on stand alone rocker studs too. I don't think there is any need to make specialty tools.
Update, I've included a picture of the compressor I have.