Its very much similar yes to that (have a look here: http://www.fightingrobots.co.uk/thre...d-diary/page15 for pictures of the internals)...

My primary concern with the base, as I mentioned is its ability(or lack of) to take punishment from external forces rather than from forces from the ram etc. It ties all the parts together taking a major part of the load, so if the base material distorts and does not spring back, then that will be the end of it!

Ellis, titanium plate at 1.2mm will of course be ideal, but I cant find a piece 500x500 in grade 5 form...apart from that, God knows how much it'll cost me!

Alex, I could probably look elsewhere for weight reduction, but I have to be careful not to weaken the whole thing... The other parts that come to mind are the hdpe sides which are 20mm thick, I could maybe drill holes in them to half their weight (they weigh 0.5kg each)