My nephew ran out of the apparently disposable engine retainers that came with his Estes rocket. The seem to get lost after every launch. Fortunately the design is simple and I was able to print these out as a replacement. The don't come out perfect without support but they more than serve their purpose.

These fit the Estes Model 000804 - Firehawkâ„¢ rocket.

Print and use to retain the engine under the rocket. I used ABS but PLA might work. The design does "sag" a bit with no support but it doesn't have to look pretty it just has to work.