I'm building migratory covers as entrances and using a plain board or an old box with #8 hardware cloth or old bottoms with #8 hardware cloth on them for bottoms. I just put 1/4" strip glued and nailed around the outside of the migratory cover with the opening where I want it.
If you build one of these you get ventilation because it's the entrance, it's easy to make and the bottom gets much easier to make.
There are advantages to both inner cover/telescopic and migratory covers. One is that you can stack migratory covered hives up against each other. Another is the price and anther is that the migratory covers don't blow off as much because they get glued down. A telescopic doesn't get glued down, although the inner cover does.
It's nice to be able to peek in a hive through the hole in the inner cover without pulling the top.
The inner cover eliminates some condensation on the lid to have a double layer there. That way it condenses on the cover instead of the inner cover.
But to work it needs a little space on each side, which is much harder to construct. Otherwise the errant bees that crawl out the inner cover get squished and the air space on top isn't there. And if there isn't some on the bottom you squish the bees on the top bars sometimes.
But you have the same problem with the migratory cover squishing bees on the top bars unless you add a spacer to it.
That's why I like the migratory top/entrance. Solves all my problems. I get a lid, an entrance, some space over the top bars, ventilation, no clogged entrance from dead bees in the winter and it gets glued down so it won't blow off so easily. And if I close off the entrance on the bottom I get no mice, no skunks, no possums.
That's my opinion.