Get an icestick (fpga on a USB stick). You can use an entire open source tool chain (thanks to great reverse engineering work) although I would use the default tools even if it implies running Windows in a VM.

Some other boards a pretty small. I have a Xilinx Spartan; really easy to carry around.

Second the default tools. The open source tools are really cool, but especially if you're just starting out, the proprietary tools are much gentler in helping you debug your own mistakes.

I wouldn't say lattice tools are super helpful, I've spent more hours cursing icecube2 than I care to recall.

There is a design for one that will replace the cellular card. You need a BIOS firmware that will not whitelist PCI-E devices.

https://github.com/polysome/aggregate-1/blob/master/README.m...

That looks exactly like what I'm looking for. Running Coreboot would probably work with something like this so that's what I'd do if someone ever actually sold these. I assume I don't need to take it out of my laptop to use it. Hopefully I can program/flash the SD card from the system.

Not that I know of. However, there's Avnet's Spartan-6 LX9 MicroBoard evaluation kit, which has a USB-stick form factor, is inexpensive, and is compatible with the free version of Xilinx's tools.

This [0] looks good but I wouldn't call it portable. If there was a case and it had some kind of conformal coating so it doesn't expload from slight moisture then I would say it would be a very good choice.

[0] - http://dangerousprototypes.com/blog/2011/04/22/avnet-spartan...

It's my understanding that you could use emulation as well in some development environments. That would forgo all hardware/cables/kits, etc.