Cut a small tuft of sisal string and apply a blob of glue from your trusty glue gun to the bottom. Then stick it on a piece of spare card, I use the same card as for my bases. Only the quick drying adhesive of a glue gun works because if PVA or contact adhesive is used, the lot falls over before the glue dries.
When required, a sharp, angled tug will pull the tuft off along with a layer of card, thus keeping it all together. Cutting different sizes and lengths and gluing them down means that you have a ready supply of these totally indispensable items. Now you can use PVA or contact glue.
Damp is better.