Sure, you can build your own by following these instructions.
However, I’ve already done that, so it seems a shame not to share the results. It’s all zipped up and ready.
Download it (it’s about 91MB), open the zip-file and drag the usb-opensim folder to your memory stick (or actually wherever you like – any old folder will do). (Unpacked, the whole installation requires just a bit over 200MB. You’ll need more space for data and assets, of course).
When you want to run it, go into the folder, and start mowes.exe. Then run opensim.bat (which I’ve added to simplify things a tiny bit).
At the end, type shutdown into the opensim window, and wait for it to save everything. The script will prompt you to close down mowes. All saved and tucked away nicely.
The passwords it was set up with are all in passwords.txt in the usb-opensim folder, if you need them.
When you want to connect, the login URI you want to feed into the grid manager of your third-party viewer is: http://127.0.0.1:9000/
[As a side note, I’ve had a little trouble with one or two viewers giving a ‘something went wrong’ message immediately during login. If you get that, try a different TPV, and see how you go]