Bad piggies-like inventory?

Hello everyone. That's my first time here so maybe i'm posting in the wrong place. Also, sorry for my bad english.

Anyway, i'm creating a 2D game with unity where you have to drag objects from an inventory into the scene, with the limitation of a grid. Just like in bad piggies (but the grid is the complete 2d scene). The image below gives you and idea:

click/touch the object (since i need just one touch and unity allows mouse emulation on screen it's ok for android too) and drag it into the scene. I think i can do by myself the grid (just put line-like-prefabs in row and columns and where they intersect put a box collider; is that ok?).

Anyway, the problem is that i'm actually stuck with the inventory and the drag/drop system. Really reaaaally stuck. Just can't find a way to do it. No inputs inside my brain. The lightbulb doesn't turn on. Can't find even a start point.

I searched for days on other topics but looks like nothing can really help me and all the questions asks how to drag from the scene to the inventory, the inverse of what i need. Also most of them uses GUIs and i HATE them since i'm targeting the game for android and GUIs are just BAD on mobiles screens because it's an hell adjust 'em since they uses coordinates values and with bigger screens they appaers where they mustn't(even with scrip for adjust position using screen getHeight/getWidth and similiar)

I'm really really stuck and sad (well not exasperated but it's pretty frustrating). Can anyone help me please? joke or just deluded mode one if my game will be famous one day you'll get a special place on the credits*mode off*

PS: if it can be useful or needed for a concrete answer, i'm using as sprites planes with integrated a box collider (don't ask me why. i just like that way of do things)

Regards asduffo