I used 90 degree angle brackets from B&Q
I dry fitted all my panels using them. It did mean I had screw holes everywhere, but that just adds to the bond strength, as when I bonded my panels, using Upol B, it filled in all the holes from behind. I then spread what was poking out of the holes, let it set and sanded down
I think you may have a problem if you use sikaflex to trial fit your panels. It takes time to go off but when it does it really does bond well. You would have to cut them apart
Mick