Over the years, WITC has considered a couple of ways of doing this.
Stamping directly into a note pad or plain jotter ensures the correct order of finding boxes is maintained.
Its also allows for notes and scribbles to make the walk a proper letterboxing journal. However it does put the pressure on getting a good imprint first time, and it isn't so user friendly when you have or intend to find thousands of stamps.
We settled on postcards in the end, and the purchase of wooden boxes at the Meet back in the mid 90's to separate blank cards from stamped cards just complemented the choice.
At the end of the day though, we only have a small pile of postcards to show for our efforts, yet we still want to show them off.
Using large scrapbooks, we photocopy the section of the OS map we walked, highlighting the route taken, note the date, weather and distance walked (thanks GPS!) and arrange the stamps around the page(s), sticking them in.
OK, so we have 40+ scrapbooks piled in the attic, which has the fire brigaide a little nervous, but our accounts of walks completed are well recorded for the future.