I have a Spark that the front plastic protective cover that protects the infared sensor on the Spark front end keeps falling off.
Any suggestions on what  type of epoxy or glue I can use to secure it?
Thanks...  

Hi, we're sorry to know about this situation. Would it be possible if you could provide us with a screenshot of this scenario for a further check? For additional information, we don't recommend fixing the drone by our customer's end as this would void the warranty of your unit. If there will be physical damage to the drone, please send the unit int our facility for further check and also use the warranty if the drone will be covered. The repair fee would depend on the result of damage assessment and the outcome of the investigation of our designated department. If you wish to contact our support, please click the link below. Kindly keep us posted on the latest progress. Thank you for your support.

www.dji.com/support

Gorilla glue 2 part epoxy. Gives you time to work slow. Apply with toothpick all edges take your time.

I have used aquarium silicon, at my first Spark.

I agree, Gorilla Glue.  If it's out of warranty, if it's still within warranty send it in and let DJI repair it