[MrForbes]

Last year we made our chassis of MIG welded thinwall mild square steel tubing. It's strong, a bit heavy but not too bad, and it's relatively easy to build. Also it's locally available and not very expensive.

We also make the Typewriter Repairmen underwater ROV frames from steel strap, for similar reasons. A coat of spray paint keeps it from rusting.

If you do a good job designing your manipulator to be light, then a bit more weight in the chassis really doesn't hurt anything.