This worked on my M103. I put the bead of benz black sealant in the corners as required, set the gasket, then I laid a piece of thin plastic sheet on top of the gasket (with a cut-out to avoid hitting the chain). The plastic was placed just short of the corner so it wouldn't interfere with the sealant. I slid the cover in place, finger installed the bolts and the carefully pulled the plastic out. It seemed to work okay as I've had no leaks after several months.

I did a dry run before using the sealant and with only a few bolts to see how it would work first.