97 Alternator belt keeps popping off

Replaced the belt after it shredded on the firewall side, noticed I have no tensioner bolt, made tension as best I could with an engine lift, clamped alternator into place with the block-mount bolt and bracket pivot bolt. Belt slides off. Re-attached belt with hand-tight tension. Belt continues to slide off, though pulleys are all in alignment.

Available info tells me that lack of a proper tensioning device is the probable culprit, though I would appreciate any input. Anyone have experience with this?

97 base model