traffic cones + paint = dressage letters :)
Have you SEEN the price of a single traffic cone though?! They're like 17 bucks each. As opposed to something like 80 cents for a plastic bucket. Or free with a heap of water if you get those big plastic water bottles, the 10L ones with bottled water in them, and use those. I've seen people use plastic jerry cans too [if anyone isn't familiar with the term, it's basically an Aussie-ism for gas can]
I need some way to mark an arena [for corners at least] and some letters, granted we can school a few figures here and there and movements at home but it's darn difficult to practice, for example, a trot leg yield from the 3/4 line to B [or E depending on which rein you're on] then canter a 20m circle! That not only is a movement in a test, it encourages correctness in the leg yield, so it's a pretty important set of movements to train. Pickets and rope sounds like a viable option to mark out the arena.
At my pony club they have proper letters for shows but they also have cheap letters for meetings. Some are made of large plastic water bottles, you know, the 10L kind you get at the supermarket? And others aren't as safe, and are made out of metal bent into a V shape so it stands up. I've seen the V shape metal ones at a few clubs now and while, given the choice, I personally don't like to ride in an arena marked out with them, they do work. And the insurance company doesn't throw a fit over them, so the risk level can't be THAT high. However if the horses are going to be in the paddock with the markers then metal is not your best choice.
Blocks of wood work, as do wide planks [one simple butt join and a lick of paint and there you have it, one dressage marker].
To make sure you know which letters go where there are heaps of diagrams, just google search :)