Sash cord or shock cord (what I used) about 1/4" thick (check mearurement) stuffed hard in that channel that the lip goes into, will keep it from slipping out. You can compress that channel with a mallet (probably the "correct " way) but it makes it really difficult to remove the seat covers later if you want .

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)