This is going to seem nuts (it does to me!) but I figured that on a digital, the 50 could afford a deeper hood than can be used for film, so I tried threading on...
THE METAL HOOD FOR A TAKUMAR 135/3.5, 150/4, 200/5.6.
(as seen on this randomly grapped link:
http://www.ritzcam.com/catalog/images/PtxSupTk_135.jpg)
Ironically, I had this lens hood before I ever went Pentax, for one of the lenses of my Mamiya C3 TLR.
Maybe I'm blind (I figure I must be--this can't possibly work, right?) but I have not perceived vignetting.
Of course the length of the lens is doubled with this hood. And it only just this instant occured to me that maybe I could get a cap that fits over the hood. Obviously, you can't put the regular cap on with this mounted! But it's so deep I feel pretty safe carrying the lens with no cap (though I do have a UV filter on, so I'm not taking much of a gamble).