In a message dated 97-09-20 08:44:54 EDT, you write:
 
<< Most of my inner covers are homemade, and very simple.  I use 3/8"
 exterior-grade plywood, and nail/glue a 3/8" rim around the upper surface.
 (The lower surface is just flat.)  Then I drill a 2" diameter hole in the
 center, for feeding, venting, or whatever.
  >>
Mine are made from cheesy 1/4" luan and similar plywood that I get free.
The trick to keeping them sound so far has been a rim of real wood on both
sides with the nails clinched from both sides. Use nails long enough to go
throug all three pieces and then some and then bend them over. Half the nails
driven from one side half from the other. Glue doesn't hurt either I have
been using tightbond 2..
 
Am gonna try the petroleum jelly though.