Don't take the sealant choice lightly. This is a very specific application for sealant. The key is that the sealant has to be slimey slick so that you can slide the cover rearward without displacing the seal, but it has to have some body to it so that it will goop up in the corners.

I'm sure that there are other sealants that will work, but you would have to experiment to find it.

When starting the shaft seal over the camshaft, use your fingernail to ensure that the seal does not "fold" and slips over the shaft properly. Put a light coat of oil on the sealing surface of the camshaft before doing this.

Good luck,