How can I create an object that fits in the gap of another object? I decided I wanted slots in the checkpoints for a number plate to
slide through and it to hold. It is a checkpoint for a mini drone
race I am doing. I basically want the "T" looking piece to be a gap
in the circle and I will need to create a plate that will fit in the
gap. Can anyone help me figure out how to join the 2 rectangles and
make them a gap inside the circle.

Also, someone mentioned the print would be too small. I could scale
it in simplify 3d, but is there a way to scale it using blender?
Also, Is there any way to measure the gap so the sliding in pieces
fit tight? I downloaded the plug-in "Measure it" but am having
issues figuring it out. It's a lot to take in, but with some
previous help I am learning a lot and hopefully one day be able to
help someone else out with a problem! Thanks
This type of cutting actually doesn't require doing a boolean. You can use Knife Project instead.

First, using your T-shaped piece as a starting point, model a flat piece that is the same shape when viewed from the side. You can snap to verts while constraining the axes to do this.



Then, separate this into its own object. This object will cut into your circular one, so first select the cutter object and then Shift-select the circular one that will be cut (order of selection matters).



Tab into Edit Mode and Hide the parts of the mesh you won't be cutting (the T-shaped piece). 



Select All, view from the side, and click Knife Project. Enable the Cut Through option in the Operator Panel.







You will end up with a nice clean cut in your mesh.



You can then delete these faces and fill them in however you like.