Neat concept, would love to see it fleshed out more with puzzles etc., but I totally get running out of time during a game jam :)
Some things that I think would improve the experience would be:
- Moving the piece selection buttons to the top of the screen so that they aren’t covering where the player goes (the character got stuck on something behind one once and it took me a while to figure out what had happened).
- Making the pieces snap to the grid might make the final level players create feel more consistent with the pre-built parts of the level.
- Working on the character’s collider, in particular its interaction with platform corners. Also I’m not sure if the wall-jumping was intentional or not, but it was cool, so maybe some animations to make that clearer.
All things considered, a neat little sandbox and I hope you keep working on it :)