Just posted this elsewhere and realized I never really showed this to anyone... maybe someone here has already done it, but anyhow.
I've been doing this for a good while ever since I swapped the lens hood on my FA43 to use a DA40-style low-profile hood.
This is simply the original FA43 hood and cap - matches perfectly - threaded onto the end of the GH-3 extension tube for the GR.
Much much more low profile and nice looking than the bulbous rectangle hood that comes with the GR setup. I often have the GH-3 mounted or at least nearby for using the 21mm conversion... perhaps not "often" but it isn't uncommon. The Pentax hood is just so much nicer and since it has a cap it of course helps prevent sensor-dust issues (of which I've still never had a problem). Doesn't produce any vignetting that I've yet to notice on any shot and is just generally smaller and better. Thought I'd share. Nothing special - just a matching 49mm filter-thread. Works great - it's light and sturdy.