If you have an iPad, consider using using that charger for your rMB as well. It's small and light and easier to carry than the 29W adapter. I purchased a $10 USB-A to USB-C cable on Amazon.com to use with the iPad charger. It won't charge the rMB as quickly as the 29W version, but it does the job in most of my uses.
Of course, this won't work if you don't have an iPad charger, or if you use a dongle to add other USB-A devices that draw additional power.