Welding rods made from scraps and a flat tipped soldering gun were my plastic fixing essentials for many years. This hokey method works to some degree. Awhile ago I bought various materials and a Model 5600HT Mini-Weld Model 6 Airless Plastic Welder from Urethane Supply Company
www.urethanesupply.com A book was purchased from a local store, too. It is published by Whitehorse Press and it is "How to Repair Plastic Bodywork" by Kurt Lammon ISBN 1-884313-37-X. Proper tools and materials make for an easier and better quality job. They are worth the cost.
The first task is to identify the plastic. Some, like polyethylene, and easy to recognize. This airbox plastic is not. It could be one of many types. It is time for the melt and sniff test. Dents are melted in an unobtrusive area of the airbox. The melting plastic has a distinctive smell. Various welding rods are melted, too. The odors are compared. Melting airbox and TPO rod have a similar stink. The plastic is identified. It is a good idea to have a full selection of welding rods for identification purposes.
A weld contaminated by the wrong plastic can be weak or discolored. The welder tip is cleaned by brush and drill and Gretchen is showing how. The welder is cold so she will not be burned. In practice it is hot when cleaned.