any idea how I can make a shallow, round (1.5 metre diametre) trough that will hold water?

Question

It needs to be either clear or metal and must be able to hold a small amount of water. only needs to be a few inches deep. thanks.

Answer 1

get a kiddie pool and paint it metallic

Answer 2

1.5 meters is a bit under 5 feet, which is a really awkward size. Getting a sheet of plexiglass 5 feet wide is usually a special order. The circumference would be 4.7+ meters, but if you bought the wide plex, you could get strips out of the left overs - a plastic supply company like Regal Plastics would certainly discuss cutting a 59" circle from a 5x8' sheet and cut 3 or 4 5' long strips 3-4 inches wide off the end. 1/8" (3mm) plex could be curved to this radius without heating 3/16 or 1/4 (6mm) would probably require heating and since few ovens are 5' long, you might have to do it in half length sections, but it only requires heating to about 240F (115C) to soften for easy curving. Assemble with acrylic cement or E-6000 or Marine Goop.
Any sheet metal shop should be able to make one if you will accept a seam or they have wide metal (4' is usual max.) What you want, from their point of view is a giant round drain pan like would be put under a boiler or hot water heater.