Does this cap require any drilling or frame modification?

No. This cap mounts at factory tailgate attachment points — your truck bed stays untouched. Test-fit first to confirm alignment before final installation.

What prep work is needed before install?

Clean the mounting surfaces on your tailgate and bed rails so fasteners seat properly. If you're painting the cap to match your truck, sand and prime first — then apply your finish before installation.

Will this seal completely, or will water still get into the bed?

This cap covers the opening and reduces debris entry, but it's not a tonneau cover with a watertight gasket. Water can still find its way in at seams — if you need complete weatherproofing, a tonneau or tonneau cover is your next step.