|The Black Arts article was written about a year after the Sikaflex product appeared. The manufacturer probably revised its instructions based on experience in the field some time after we did our work, because I don't remember any mention of spacers. About five years later, both are still intact.
Both Kathy and I were re-mounting flat pieces of plexi, so if you are careful, I can't see that spacers are essential. On the other hand, I can see that if you were mounting a curved piece, such as the side windows, exerting an even pressure without them could be difficult. There would be a tendency to squeeze material excessively at the centre of the curve, so spacers would give assurance of a better bond.
If they don't make a specific recommendation, I'd look at those clear plastic discs or domes that hardware stores sell as bumpers for kitchen doors. They're small, they're clear and they're self-adhesive and so easy to handle.
I don't think I could hold something absolutely immobile for 10 minutes; I could, however, make the framing in 10 minutes, and apply it in one or two. It's insurance that the plexi won't move and spoil the bond; your decision.
Please let us know how it works out so the article can be revised if necessary.
C&C 27 Mk IV