Ideas for XL pot saucers?

[Dimples]

I need some extra large pot saucers for some outdoor planters, but regular storebought ones are out of budget considering how many I need (they're over $30 each and I need eleven ). Thought you all could help me brainstorm some alternative items that would be cheaper and will work for outdoor use without deteriorating from exposure in a year.

Diameter: 28-34 inches / 71-86 cm
Depth: 2-4 inches / 5-10 cm, but I'm happy to cut down the sides of deeper items so long as it can be done with a razor blade or tin snips/Cutco sheers.
Other: needs to hold water and withstand southern California sun year-round. Otherwise, I don't really care what they look like. They don't even need to be round, square would work too.

Thanks!