Creating a Custom Slotted Miniature Base
Take an image of the base of a model (from underneath the model). Make sure the image has a distinct difference between the background and the base.
Use the Photoshop Lasso tool to select the base. Color it white. Then right click on the image and choose "Select Inverse". Then use the eraser tool to erase the background. Right click again and select inverse (to select the base again) and then go to Edit > Crop in order to shrink the image down to be just the base. Here is an example of a the result. I added a black background so you could see the image.
Drop this png into imagetostl.com to turn it into a 3 dimensional STL file.
Download the STL and then upload it into TinkerCAD.
Now drop in a base with the width and length you want:
Measure the width of the actual miniature's base in millimeters. Then, shift click on a corner in TinkerCAD and set the width to the measured width. This will scale the whole 3D model accordingly. I usually add 1 millimeter to make sure the miniature will fit. Also, use the "reflect" tool to flip it. Since you took a picture of the bottom of the base, you need to flip the object so that it creates the correctly shaped hole.
Now, make the other shape a "hole":
You will need to move the terrain piece up 3mm so that it is resting on the disc. Then, move the hole up until it cuts enough out of the terrain for the model to fit into, but not too much so that it would be hard to get the base to fit. I usually drop in 4mm down into the terrain piece for enough of a groove for the model to snuggly fit into, but not so deep a cut that it's hard to get the model in or out. Finally, center align all three objects as shown below.
Join the three shapes: