This is a spacer/adapter for the UpLULA when using it to load the S&W M&P Shield 9mm and .40cal magazines. Without this spacer/adapter, the Shield's single-stack magazine shifts/wobbles while loading - making it annoying and cumbersome compared to loading the Shield magazines with the spacer/adapter in place.
The spacer/adapter was created in Fusion360 and supports the full length of the magazine inside the UpLULA. It has a nice friction fit that stays in place while loading, yet it's still easy to remove when you switch back to double-stack magazines.
No supports are needed if you print it base-down. It's very sturdy with 3 walls and 35% infill.