Hi Brad,
How about “sharpening” the foot of the rubber bump stop. Carefully trim the food on a bench grinder, into an arrow head shape. Put rubber grease on the sharpened foot and put the rubber in position. Now fill the space between the rubber buffer and the suspension cradle with wooden blocks whilst holding the buffer in position and with a little tention on the string, you mentioned before.
Now jack up the suspension on that side. The rubber bump stop will now be squeezed between the wooden blocks and the suspension.
The rubber grease, pressure from above and pulling on the string, might help the foot of the rubber bump stop to slip into place.
Chris🤞🏼🤞🏼