As it happens, I 'invented' (OK, Wombled
) something very similar myself 6 months ago, modelled on a pheasant feeder I'd seen - I didn't realise until later that there was already a commercial version available for chooks.
I'm using it for layer's pellets now. It was a bit fiddly to set up, but once I'd got the hole size right so that only a couple of pellets are released per peck, it's worked perfectly. There are more details on how I made it on
my wee neglected wordpress blog. The trigger is actually quite stiff once feed is packed around it, so there are no problems with wind etc. It's also too high for rats etc to get at (I think - to my knowledge we don't have any here).
If you've already got a suitable barrel or bucket, you should be able to knock one up out of bits and pieces you have lying around. Just start with the hole too small and then enlarge it bit by bit until you've got it right. The slabs underneath are also important to stop the area from becoming a mudbath.
Hope that helps!