Open TABS, create the unit you wish to be a different base.
Now right click on TABS in your Steam library and left click manage, then left click browse local files.
Next open TotallyAccurateBattleSimulator_Data, then CustomContent, CustomUnits.
Sort by recent, and select the top file, open the .unit file with any text or code editor of your choice (for example, notepad on windows).

Now replace:
With:
For example, the snake:

Please note that if you make any edits to the unit afterward, it'll reset the unit base, so I recommend making changes to your unit in the creator first.

Note: This is not all the IDs. Here's a list of known IDs as of 09/05/2022.[totally-accurate-battle-simulator.fandom.com]

For more advanced customization, here are two other guides which are worth a look at:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2417907636
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2807213968