I trim the edges or rim off each prop tray that I use. I have to, to make my "cotton flannel pad" method of bottom-watering work.
Plug trays seem rigid enough not to need the rims (especially once cut into slices or blocks).
They are also rigid enough that they don't cut easily. I have a pair of scissors with thin, long blades, which I sharpen as needed.
I can cut plug trays with those by pushing the scissor-tips 1-2 cells ahead of the cutting point, and only cutting about one cell-width before sliding the scissors forward. The cells ahead of the cutting point keep the scissors "lined up". Otherwise I would cut down into the sides of cells and mess them up.
I've tried to cut plug trays with various knives, unsuccessfully. Probably the "blade' attachment on my wood-burning tool would work, but the scissors go pretty fast and do a clean job as long as I "take small bites".
Tin snips would work even better, but mine are too thick to fit between cells.
Bad rims on a flimsy 1206 six-pack insert
OK rims - cut off of a flimsy 1206 six-pack insert.
Note that the cut slits press down ON the flannel so that soil can touch the cotton fuzz and make a capillary connection.
4" pots, one 1206 six-pack insert and two rows of an 8x16 plug tray of 128 cells .
The six-pack should have been trimmed closer.
