Around 400 unit of the Projector, Rocket 3-inch, No 8 Mk 1 (“Land Mattress”) have been produced in United Kingdom. British and Canadian forces used it form 1944 to the end of WW2. It could fire his destructive payload in around 7 seconds.
This model doesn’t required supports to be print. However, it is recommend to use a raft for the wheel shaft and the rocket tail. Assembly required.