When photographing some insects that are dangerous(such as brown recluse spiders) I keep them in drinking glasses to allow me to light well, and restrict them. This works well because the camera can go into the glass also and get very close for macro.

However, sometimes I want to do other than top down shots, so I have to set the glass on it's side and put the camera parallel to the table. This leaves a small gap, and certain adventurous critters can escape, crawling onto my camera and sometimes out. To avoid this I have used a dirty sock to wrap around my lens, making it closer to the width of the opening so the spider is sealed in.

Another solution is to use a cloth coffee cozi on my lens.

I will post a picture of the setup later, and a picture shot with it.