For those looking to build your own - its not that difficult. Take as long/short as you want. Some of these ideas took people a few months with a few minutes/hours here/there - others were done in 2-3 hours.
Did several hours of accumulated hours "lurking" the BGE forums for thoughts and ideas on building my own cart for a BGE. To search for older posts, search on "DIY Cart", "whiz cart" or just "cart" should catch 80-90% of the posts.
Many ideas out there, my assessment is there's not a lot to do wrong!
Many many good ideas, so just pick one and run with it is my suggestion.
I bought my large egg two weeks ago, and built my cart the day prior.
Cart is "temporary" since I am designing and building a BBQ island for long term use and enjoyment. I just wanted a cart for the next 2-4 months, but it needed to be sturdy enough to be moved a lot while my BBQ island is "under construction".
Lumber was taken from a redwood fish tank stand I had built 6 years ago, and decommissioned earlier this year. 240 gallon fish tank stand provided plenty of lumber, and I re-used the wood screws.
Design is basically from the BGE web site, with the four main vertical supports I converted into 4x4's (two 2"x4" screwed together) for added support. Wheel design I borrowed from Whiz' site. Took me 6 hours to take the old stand apart (I forgot I used the whole box of deck screws!) and cut and assemble the new egg cart, including two one-hour breaks for the rechargable drill/saw batteries to recharge.
She may not be purty, but she's as solid as a brick sh*t-house! Its sturdy enough to get up and dance on.