I was going to suggest, if you're randomly determining terrain as the party explores, have 1 example of each terrain type on the GM layer of your map. Once the party enters a new hex and you check to see what is the terrain, then copy that hex type from the GM layer and move it to the map layer. You could also do something similar using the Journal tab in Roll20: Create a "Character" entry for each terrain type, making sure to pre-size the on-map token for the hex size you're using. Then when the players explore a new hex and you determine the terrain for it, just drag the "Character" for that terrain type onto the map and fit it into place. Be sure to do it on the Map layer or course... or else, don't forget to send the terrain token to the Map layer, once you get it onto the board. These are just a couple of ideas that immediately come to mind... others who are far more clever than me will undoubtedly come up with other suggestions.