I had asked a question that nobody could answer ... but I went ahead and figured it out myself. My question was: if I could NOT damage the new windowsill with screws but use 3M adhesive strips instead. The answer is YES. I removed the Velcro strip from the board and added 2 complete medium size 3M Velcro strips PER HALF of the board. The velcro strips are important because they’re thicker and due to the hardware for folding this shelf in half, it doesn’t lay flat on the sill. Then, on the outer corners, I used 2 large 3M foam strips PER HALF. On the outer corners, the board lays flatter to the sill and the foam touch’s and works well. That’s said ... my entire perch has 4x medium Velcro 3M strips and 4x large foam 3M strips. I did not use the screws as this is a new home and I didn’t want to damage. (Excuse the construction background in the photo)

Both my cats lay up there at the same time, and it is secure.