how to strap something to roof of car without a rack

Short answer: don’t—longer answer: if you absolutely must, do it properly.

Use a purpose-made soft roof rack or dense foam blocks, a protective blanket, and proper cam-buckle straps; never bungee cords, and route straps through the door openings, not the windows.

Keep the load light and flat, under your car’s roof load limit (often 50–100 kg dynamic), spread the weight evenly, and stop after 10 km to recheck tension.

Avoid ratchet straps on thin roofs, keep clear of curtain airbags and sunroofs, and don’t let the load block your mirrors or vision.

Drive slower than usual, listen for movement like a paranoid meerkat, and if there’s any overhang, fit a bright flag by day and a red light at night as your state laws require.

Honestly, a proper rack or a small trailer is safer—and cheaper than repairing a creased roof and wounded pride.