If you are going to be placing the tortoise in an educational display, my opinion would be: don't even consider something like a tortoise diaper. This would not be at all educational but more circus like and would also promote ridiculous handling of pets.
It would be better to exercise your tortoises before transporting them, then soaking them for a short while. That usually does the trick. if it does not, then just clean up any mess they make. One out of the enclosure way to display them at an educational event is to place them atop heavy paper such as construction paper (available at Home depot in fairly large rolls, looks like brown wrapping paper). If they poop on that, the kids will all get a good laugh while seeing a natural function. Then you can clean it up by simply rolloing up the paper. You will need something to hold the paper down at its corners and along the edges, you can use that sticky blue stuff kids use to hang posters on walls, or you can use a few school books.
If you are worried about them defecating in their carrier, use an ample amount of bedding material in the carrier while transporting them. Make sure to also have a trash bag into which to empty any soiled bedding. Carry extra bedding, paper towels, and a spray bottle of water, and you can have it cleaned and refresed in only a couple of minutes.
best regards,
Glenn B