Mike Lindsay wrote:
Many years ago I had the same thought. I made up a reflector from 3
polystyrene ceiling tiles arranged at right angles to each other,
and
covered with kitchen foil. (If you arrange mirrors like this a light
ray striking it will be sent back from where it came, it works with
radar too.)

Then I found there wasn't room in the glider for it.

Maritime radar reflectors are available, they are also too big...

Mike,
I think Henk was talking about a reflector for sunlight, not radar
transmissions.
This means he needs to have the reflector on the outside of the glider
so it can be seen by others.
The main problem seems to be the drag issue, and then whether it is
any use anyway.
IMO unless very large, it won't be visible at any useful distance.
Even strobe lights aren't that good in bright sunshine after about
half a kilometer.
Cheers, John G.