Here's a 4x8 machine done with Bishop Wisecarver bearings that seem quite reasonable in price and timing belts for action instead of rack and pinion (much better value from what I can find of R&P at mcmaster) so its one example of a way to do it. The bwc.com bearing rails are available to like 20' so they have your dimensions.
Although I have yet to build my second machine as well, I'm thinking of putting my THK rails back up on ebay and switching to the BWC bearings instead. They'll hold up better in the dust (no grease needed like THKs) and will cheaper in a larger size. So far just playing around with kevlar reinforced urethane timing belts from McMaster has been quite positive, so that seems to be my likely motion source too.
Here's also a couple places with plans, seem to be ok from what I've read on here but no personal exp.