My question is: does anyone know what the mesh size should be of the screen so the eggs could fall through?
Lee, I used the screen and shelves method for a couple years. So can help you but I stopped using them. The screen and shelves are simply more labor intensive. In the long run it's extra labor that didn’t need to expended in the first place. A lot of people quit after a short while because the farm is just too much work. So simpler is better.
That said I have no room to talk. I did start with the more complicated system first. I eventually changed over to a sweater box and am quite happy with it. The box takes nearly no work at all. It’s as simple as adding some moisture as needed when collecting worms . And worry about cleaning the frass out when the feed is gone, about once a year.
Anyway I’m off my soap box now so to answer your question and more you didn’t know to ask.
I’m pasting in a copy of directions I wrote up a couple years ago for a drawer system. Hope this helps.
Use metal window screen. Has to be metal the worms can chew through the cloth screening. Glue the screen on the inside of the container. If you put it on the outside it will come off.
Use all-purpose hot glue and make sure it is hot as possible when applied (sticks better). Don’t remove any hot glue that drips out onto the screen. The hot glue de-tempers the wire enough that the worms can chew through it after a few weeks. So the dripped glue acts as a seal over the damaged spot.
You will need to check the screen for bad spots in about a year then every few months after that. You can replace the screen, cut it out and put in a new screen. You can even use the glue gun like a sodering iron and melt the new screen into the old glue.
When preparing the drawer leave 1 to 1 ½ inches of plastic all the way around the outside edge. You will need the lip to glue onto and it helps support the bedding. The actual size of the openings depends on how wide the shelving supports are below the drawer. If there is an opening over the drawer supports the feed and baby worms will build up on the shelf support and be pull outside the drawer every time the drawer is opened. Pay attention to the drawers themselves, sometimes the drawers have a lip at the top that will need to compensated for. It helps if you take a sharpie and draw on the drawer all the areas that has something under the drawer then plan the openings in the remaining space.
Now for cutting the holeS. After some time the screen will start to sag and drag on the shelving opening. So it’s wise to plan for this. For more support under the screen and to make it easier to open and close the drawer narrow lengthwise bracing helps the drawer slide in and out easer. Cut out 2-3 lengthwise rectangle holes (2 to 3 inches wide) for the length of the drawer. Leave at least an inch wide plastic strip to help support the bedding between the holes (WIDER IS BETTER). The width of the hole you cut out depends on the width of the unit you bought. Try to avoid a brace that goes side to side. Lengthwise bracing is far harder to cut but they do keep the screen from dragging on the shelve supports. Sideways braces catches on the edge of shelving under the drawer, thumping or stopping the slide altogether, making a mess every time the drawer is closed.
To cut the drawer plastic without breaking it you will need to use a box knife. Using very gentle pressure cut very shallow groves and go over it repeatedly until you have cut all the way through.