A subframe for a motorcycle. I only weigh about 220, add another 10-15 lbs with gear, and then assume there will be bumps in the road.

I still intend to print the bulk of it, just reinforce it with aluminum. It will all be visible, and custom is cool. Plus I need to do up all sorts of brackets and stuff for wires, tail light, various electrical control bits, battery, and all that.

I see no problem just designing it with a channel all the way around to just drop in square aluminum tube from the bottom, then have the printed part just "sit" on top of it.