Sounds like you would be better off just buying a set of good headphones and taking them with you when you leave.
A thief will cut into your soft top to steal a cheapo amp just the same as they would if you had more costly gear. They don't know the difference until the damage is already done. The key to not having your stuff stolen is to not let anyone know it's there. Stealth install methods, no windshield sticks and not bumping within a couple of blocks of where you live or places you regularly visit is a good start.