I made this one the hard way with each rectangle a separate piece. Long, time consuming, weld seams were required. It was made from 7 gauge scrap which I had on hand. It's probably nearly bullet proof for puncture but a much cheaper version could be cut from a pattern and formed with a break to reduce the welding a lot. Probably 14 gauge would actually have been thick enough in my studio.
This was built on the table top and fitted into a base bracket and secured at the top with two bolts. This quick removal/replacement was designed to accommodate leak repair and it proved handy immediately as a small leak was found shortly after initial testing.
I got a lot of advice on the desirable reservoir capacity. I ended up making a tank with over 10 gallon volume and I think this is over done. I understand that a large volume is favorable for keeping the fluid cool but I operate my press with long intermissions so I don't think heat buildup would ever be a problem with a reservoir half the size. Also, I would position it so that there was no obstruction to the working area. That would occur automatically if a "C" frame design was built.