To make your own casters you need
a large flat bottomed pan or plastic box.
A riddle, with holes large enough to allow the maggots to crawl through
but small enough to stop casters.
Some dry ground bait or saw dust
and of course some maggots.
I use left over maggots from a fishing trip
The same ground bait mix I use when fishing
and a cooking Wok.
I put the maggots in the Wok with enough
dry ground bait mix to cover them.
I leave them in the the dry shed at normal air temperature.
When the casters start forming I run the Maggot / Ground bait mix
through the riddle.
(See Photo below)
When all the maggots have gone through the riddle
your left with the casters.
I then pick out any dead maggots, feathers and other bits,
leaving clean maggots.
If they are light coloured I allow them to darken off
a little but not to dark.
Then they are bagged up and as air as possible taken out.
and the bag tied up. Then put them in the fridge till you need them.
(As in the photo below.)
There you are nice fresh clean CASTERS