you know, this experment if we work together.
All we need is to find where people are in the world, and get several of them to work together.
Firstly, we need the right people. if we work out where we are on a Real Earth Map, sorry, Round Earth Map, then courtesy of the wonder of the internet, we can team up people who are in the same longitude - for example, Finland, estonia, Greece, and South africa, or Equador, Florida, NY state, and Quebec.
the material costs are, pretty much, about $5/?5 at most - in reality, probably less..
get four 2m long canes from a garden centre, get some string, and a bolt.
use some of the string to tie three canes together into a tripod, and set it up on an open flattish area with no buildings to block the sun.
take the bolt, tie it to the other bit of string, and make a plumb-line. drop it slowly from the point of intersection down to the ground level, taking care to make sure it is steady, and not swinging like a pendulum.
the point of the tripod, and the point on the ground level are, for all intents and purposes, perpendicular to the normal of the plane, regardless of any slight irregularity in the ground. (obviously, if you're trying to put this up on a 45 degree hillside, you're a little bit "special", so try to aim for as flat a ground as you can find.)
now take the 4th cane, and firmly drive it into the ground from the the plumb-line point and going up to the top of the tripod. you've got a vertical stick which is accurate regardless of the surface. unless you happen to be standing 10 m away from a 500 cubic foot block of pure uranium. in whihc case a flat earth is the least of your worries.
now, at midday on the same day, we measure the shadow and report back with location, altitude of the location, and the exact length of the stick, and length of the shadow. direction of the shadow too.
because there are now more than two points of reference, we can start to look at using the known distances between these points, and calculating the distances, on the same day, at the same time. if we use a real-time chat programme like IRC, or MSN, it would infact be possible to co-ordinate it to "now!" and measure at that point, so we dont have to worry about differences of being within a similar time zone, but being in slightly different longitudes. if we get people in different longitudes, who are to either side of the midday line, we can work out if the curviture is 2 or 3 dimensional. (cylinder or sphere)
and therfore we can have two absolute answers. as the multiple points give us three angles, we can triangulate with the distance and with our own data, (which should demonstrate different altitudes for the sun if it were a flat earth) we can conclude either a curvature, or that bendy light is infact 'real'.