I recently made (but have yet to use) a water gauge using 1/4" clear
neoprene tubing bought at Home Depot hardware store for about 25 cents
a foot.  I also bought a 6 foot 1"x4" pine board.

I came home and cut two parallel grooves in the board, just slightly
smaller than the outer diameter of the tubing, on a neighbor's table
saw.  A wood chisel was applied to one end of the board, making a
rounded passage for the tubing to go from the top of one groove, around
the bottom corner groove and back out the other end at the top.  The
excess tubing was allowed to hang off the top and it will be connected
to a water trap (made from a pickle jar, two 9/64th" nipples and some
silicone cement), and thence to the device to be measured.

I mounted the board, upright, on a piece of scrap 2"X8" after painting
it white with a spray can.  I then marked off the center, between the
two tubes, in inches, making a wider line every ten, since that's how
my brain works.  When I get ready to use it I'll put some water mixed
with food coloring in the tube.  I think the whole thing cost me about
$10 and a little time.

John McClelland