I hit the store to buy a ceiling mount for my Optoma HD66, but the store was out of stock. So I designed up this one, and printed it out. It's designed specifically for the HD66's slightly odd 100 x 100 x 110 mm triangular M3 mount, but a redesigned ProjectorPlate.stl will let you stick any projector on there. Swivels in XX, YY and ZZ, and is fairly stable too.
Print it out fairly densely, bolt it up with M5 bolts (Darwin leftovers), and add some fender washers to spread out the loads. Tighten up the joints until you have a nice level of friction. Nyloc nuts help a lot.
Don't forget a safety leash, in case of total failure - I use 650 lb kite line, should do the trick.