Use something that presses down the entire sheet, not just part of it. A simple approach is to place a larger flat object, such as a book bigger than the page, on top so the whole page is held against the glass. You may not be able to fully close the lid, but the scanner can still work that way. Just use moderate weight so you don’t risk damaging the scanner glass.

Another option is to scan with a single loose sheet and lower the lid fully so the lid provides even pressure across the page.

The key is even pressure over the full page area; small objects can leave parts of the sheet raised, which makes stitched scans harder to align.