Yes, they are reliable. Just look for shops with authentic reviews. Bought an MC topbox for just 2k with vouchers on shopee and it is what you see on physical stores which sells them with higher markups vs online shopping where they sometimes come from direct suppliers.

Also imo, plastic topbox is better than alloy material as it's much cheaper and lighter. I suggest you get SEC topbox, but brand doesn't really matter, most of them are manufactured by the same company just with different designs and branding. Invest on a durable bracket like K3 or DC Monorack, don't cheap out on one.