I plan on using 1/4" plywood for thinner covers, does the "framing" mount directly on the plywood? also on the outer cover, does the 3/4" ply mount directly on the top of the edge pieces or should "rabbits" be cut along the frame edges. thanks
They can be done a lot of ways. Some inner covers are a frame with a groove that leaves about 1/8" space on the bottom and 1/4" to 3/8" space on the top. Some are flat on the bottom with a space on the top. They all work more or less depending. The flat ones don't work so well on my top box of PermaComb because the spacer picks up the frame about 1/8" so they burr it.
Personally, I either make a vent cover, which is my design, or a migratory cover, which is even easier. If you have scrap 3/4" or 1/2" plywood you can just cut it 21 1/2" x 16 1/4" and put a 16 1/4" long by 3/4" wide by 1 1/2" cleat on each end.
On the inner covers you will get a stonger job if you dado a groove in the frames and mount them as described above, bee space on each side.
On the outer cover, if it is a telescoping cover with a metal top it will work a little better if you rabbet the frame and set the plywood in the rabbets, then fold the metal over the edges. Stronger, more rigid and puts the metal down on the rim with less foldover.
You don't need an inner cover if you go with the migratory tops but I really prefer the telescoping cover.
Advantages of inner cover/telescopic cover:
If you build it as described with some space on both sides you get a nicer space on the top bars.
The inner cover makes an airspace for conensation between the inside and the outer cover so it keeps it away from the bees.
The inner cover allows you to put a puff of smoke in the top before you pop the inner cover loose.
The inner cover allows you to peek inside and not open at all sometimes.
You can easily put a notch in the inner cover and make a top vent/entrance.
You can add a popsicle stick on top of the inner cover and make more ventilation.
Disadvantages of a inner cover/telescopic cover:
More expensive to buy or make.
While the inner cover will get glued down, the outer cover will often blow off unless you put something on it for weight. Around here it requires a concrete block to keep it on.
One more piece of equipment to keep track of
Advantages of a migratory cover:
One less piece of equipment to keep track of.
Get's glued down and doesn't require a weight on top (less work not having to lift a concrete block off and back on)
Allows you to put hives right up against each other because it doesn't protrude on the sides.