Gary
I thought the 'Level-up' only came in one size, it's the smooth ramps that differ (I think?).
The level-up is the stepped version which you can then buy a locking chock so you don't run back down the steps - there's some for sale on ebay at around £18pr. I just thought I would post a wanted ad' on the off chance of someone having an odd one.
I think an alloy one would be a lot of work, and would not be any lighter than the plastic ones... but would be stronger!