There are a lot of factors which go into choosing an enclosed trailer. You may get more response if you provide details on what is important to you.
- Will your trailer be used only for Amphicar?
- How often will you use it?
- What type of tow vehicle do you have? This will significantly limit or expand your options.
- Do you want the best money can buy, the cheapest that will get the job done?
Aluminum trailers are much lighter than steel, but also more expensive.
If it is only for Amphicar you likely want a smaller trailer. However, don't go too small. I had one which Amphi just barely fit in. Loading and unloading was difficult as you had only an inch or so on any side. It also made tying down a pain, you could not carry accessories, and even getting in and out of the car was difficult. On the other hand, an oversized trailer will use.more fuel, requires a bigger tow vehicle, is harder to maneuver, etc.