Now find a scrap piece of board to stiffen up the pipe
half.
A 1x4 or a piece of plywood or panel board,
even a 2x4 will work but it tends to make the sluice
a bit bulky.
Now after you have found a board you think will fit check its size next to the pipe half.
Do this by placing the pipe cut side down and placing
the board on top of it.
The board should go the whole length of the pipe half,
give or take an inch.
The width isn't that critical, but around the same size
as the pipe works best.