I've been looking more into gold trommels lately and the biggest conclusion I'm coming to is that while yes, you can build your own and yes, they may work just fine for YOUR needs... it's just that. Nobody's able to build something that works as well as a finished product unfortunately. :/
I'm definitely no expert since this is really just a hobby of mine but it sounds to me like a mountain Goat trommel is what you need. Light, portable, versatile.. it's durable but it's no heavy-duty machinery like you mentioned.