Mark Johnson (I think) made a really nice bracket assembly. He didn't choose a plasma but his TV had the same problem -- too narrow at the top to support a center channel. If he's not off sulking about his room acoustics maybe he can point to a good pic

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8