Thanks for posting that how-to
@jim01749. That procedure makes a lot of sense, based on the window replacements I've done. I think the mounting tape might work in some cases, but could also make a huge mess when the sealant presses out onto the tape, depending on the geometry of what you're trying to install. The sealant should be applied generously for a thick (flexible) seal.
Also, the procedure discusses prepping (sanding with 80-grit sandpaper) both the fiberglass and window side before installing with sealant. I think that this step is extremely important, and might be often overlooked. The sealant needs roughened surfaces that it can mechanically hold on to. The surfaces also need to be cleaned up with a solvent, after sanding, to make sure there's no grease or sanding residue on the surface. I would reach for rubbing alcohol, which I keep on board for any time I'm bonding or taping something that needs to stick.