drywall-anchor
I have 20mm previously drilled holes in double thickness insulated plasterboard.
I think a toggle bolt will be the most appropriate, but I’m struggling to find something that will be appropriate given the thickness of the wall and the insulation.
Any recommendations welcome!
Edit: I’m hanging a 12.5kg mirror, but the large holes are already there.
You might have answered your own question... what about a (larger) needle?
Tie the fishing wire round the middle of the needle (and perhaps use a dab of glue to hold it in place if it won't hold itself).
If you can't find a large enough needle, try a small gauge knitting needle? Again it might need a dab of glue to hold the wire in place in the centre.
It's probably double 5/8ths drywall, I'd use toggle bolts. And then washers and 3" deck screws to the stud for safety (not drywall screws), even if you're going to do all four corners with toggle bolts. There should be some one-by inside the cabinet at the top (and possibly the bottom) this is where you want your fasteners penetrating, not just through the flimsy back panel. Your local hardware store should have these:
1/4" x 4" Toggle Bolt: (tannerbolt.com)
You can skimp on diameter if you want, but the shorter they are, the harder it's going to be to install all four at the same time. The size shown is my go-to.
Hand tighten only, or you risk stripping the tiny nut in the spreader or compromising the drywall integrity. You need to be able to feel what's going on in there; they should just be snug. Be extra careful with smaller diameter toggles as they have even wimpier nuts.
Other types of drywall fasteners are discussed here: What benefit do plasterboard drywall screws have?
Best Answer
These toggler anchors will work for thin to very thick walls.
You do not say what the insulation is, The insulation may be a factor in getting them to toggle into place properly. I sometimes use a wire to push fiberglall insulation aside a little so they sit properly.
I have posted several answers about installing these anchors read a few for info and tip/tricks.
https://diy.stackexchange.com/search?q=user%3A56737+toggler