your best option is to place the probe directly on the heat tape, under the enclosure, and secure it with a hot glue gun or heavy duty tape. if you take your temperature readings from there, your thermostat will have to be set to a higher temperature in order to adjust for the space between the tape and the bottom of the enclosure, but you'll avoid this problem altogether.

if you continue to have issues, perhaps others can describe in more detail where they place their probes. in any case, leaving a loose probe inside the enclosure where the snake can disturb its placement or interfere with its readings is probably a bad idea.