The 2 clips were used from the factory to set the gap correctly on bottom of the glass and there was usually a tapered rubber piece on each one. I usually just use a block of wood or rubber and cut the thickness I need to set the gap right on the bottom and the glass will rest on the spacers. You can take them back out after the glass is set. This worked for me but I’m sure there is other ways also.
When I removed my rear window,the blocks where still in place from the factory. In the glazing industry I believe the rubber blocks are referred to as setting blocks.