EDITED: 
A simple correction to the method I describe makes it much more easy:
1.  After adding the brim ears, just merge them with the object to be printed. 
(All about negative part etc, can be skipped). 

2. Even better solution: 
- place a small object (1 x 1 x 0,2) near to the corner what needs a brim ear. 
- give this object a individual brim 
- add instances where needed. 
- prusaslicer will place them very nicely. 

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And it happend on my printer too: warping. A nice print (5 hours printing time) lost because of warping. Reading/surfing the internet (forums) I found the solution to use Brim Ears. 
Piece of cake, to download one. And I tried a couple, but found that:
- when the ears overlap the first layer of the print, it looks weird. 
- when placing against the edge, it is OK, but very hard to get it just right. 
A little bit to close, and it is hard to remove. A little bit to far, does not help against warping. 

My solution
So invented myself (and maybe many others did this before I did ;-) ) to use PrusaSlicer: 
- import the model you want to print on the plate
NB: Now prepare this to be used as negative part: 
      - cut the upper part off, you just need a bit more then the height of 1 layer. 
      - check the elephant foot compensation (default 0,2 mm) 
      - calculate the size of the negative part: original size minus 2,5 x elephant foot 
         example original is 100 mm, then the negative should be 100 - (2,5 x 0,2) = 99,5mm
       - scale the part down to this size (only X and Y) 
       - save the part as STL (export plate as STL)
       - add > part > galery> import the STL and save  in the galery (to be used as negative part)
       -clean plate
- import the model you want to print again 
- -  add > part > cylinder to make 1 ear about 25 mm diameter. (my first brim ear :-)). and height equal to layer height. 
- copy paste the brim ear as many as needed and place them on the tricky places. 
-  select the ears > add > negative volume > choose the part you prepared. 
- place it exactly on the model you want to print  (use the object manipulation box!) 
The negative part will cut the ears exactly against the borders of your model. 

When printed it should be very easy to remove the ears without damage. 

Enjoy, and please feedback in case my explanation is not precise enough. 

PS: It is not possible to export part + negative volume as a combined STL, so I made an image to show this. 

Eef