Timebomb

When you create this spell, you imbue a piece of a floor, ceiling, wall, or a solid, nonmagical object with a plain surface (the body of a carriage, a table area or the like) that isn`t worn or carried, of 5 square feet with magical energy. When you give up concentration on the spell, or when the duration time ends, the magical energy erupts, forcing each creature (within 1ft) directly on top, in front, or below the area to make a Dexterity saving throw. A creature takes 2d8 force damage on a failed saving throw and is pushed away 5 feet in a direction of your choice, or half as much damage and isn´t pushed on a successful one. 
You can use Magical Tinkering to attract creatures to the timebomb on the same square. 

At Higher Levels. When you cast this spell using a spell slot of 2th level or higher, you can imbue one additional 5-feet square with energy, causing 1d8 force damage. The squares can be stacked on each other for 2d8 or more points of damage, or be placed horizontally or vertically adjacent to each other for 1d8 damage each. When you use a spell slot of 4th level or higher, the targets are pushed 10 feet, at 7th level or higher, 20 feet.