T-shirts would be great, Peter, and they're not that hard to do, I've done them many times for conservation organizations.
One comment - use a quality t-shirt. Here in the US the standard for years has been the Haines 100% pre-shrunk cotton Beefy-T ... they're more comfortable and tend to last longer than man-made fabrics or 50% blends. Don't know what the equivalent would be in Europe, though, but I'd rather spend an extra buck on a quality T rather than scrimp on quality.
