If making new x end plates sounds like more tinkering than you want, how is making a new gantry from scratch less?
If you are really set on extending the X rather than the Y, then you will need to be looking at professional machines for inspiration.on how heavy and stiff to make it. There are threads on here that show the making of some pretty hefty carriages using linear rail and such - they are completely custom machines. Edit: Here is an example: Project Log - Franken-Carve
You came here asking for advice on the simplest way to set things up with the size work area that you want. Various people here have chimed in with their thoughts and reactions. What you are looking for is not impossible, but there are reasons why the kits that are available only go to a certain size in each dimension.
The beauty of these systems and the community that has grown up around them is the diversity of people and ideas. I look forward to seeing your additions to it.