So I didn't find this in the DO NOT suggest sticky and searching found only one thread that went into little detail.
At the moment the SMG can go through all bunkers without 5 block thick walls fairly easily. This is easy to repair but there should be a way to not rebuild an entire wall because of a clip from a SMG got sprayed around there. So have stronger blocks.
Using the Q and E buttons a player can change the density of the blocks he/she places. A higher density block will cost more "points".
Your current dirt block, that can be easily damaged by bullets and destroyed by a right click spade, would cost one point.
The next layer (hardened/compacted dirt?) would be slightly more durable taking more shots or left mouse button spade attacks but still being destroyed by a RMB spade.
Sandbags would be more resistant to shots than compacted dirt but have the same spade vulnerability as normal dirt.
This would go up with the typical things such as concrete, metal and others. Some would have trade offs like sandbags having better shot protection but no spade protection while others just might be plain stronger at everything.
As making the blocks costs more breaking them would also give more giving players a reason to dig in this certain spot or as a easy way to store resources.
Each block would have different colour limits (as I don't believe textures can be implemented on voxels) so dirt could have green to brown, concrete all shades of grey, sandbags brown and yellow colours (dirtbags?) and camouflage blocks (weak to everything) could have all the colours.
It would also take some time holding down the block for it to appear completely so you can't just make a concrete bunker in the middle of a firefight magically and have to secure the location first.
Feel free to build upon this.