Can we get a portable 123D?

You can usually find the specific licenses for most apps (not all, some are a bit weird about it) by checking out any links regarding legalities, termos of service etc.

I specifically found theirs via the "Terms of Service" link at the bottom of the page, then "Software License Agreements" on the left, then "English". It is a lot of legal jargon, but usually an open source license will identify itself as such pretty quickly.

Having a quick read of that license it is a bit too restrictive for portable use. Here are the highlights:

1. The usual restriction of not being able to " translate, adapt, arrange, or create derivative works based on, or otherwise modify the Autodesk Materials for any purpose" - we can usually get around this one by getting direct permission for a portable package, or by doing an online installer. But wait there's more....
2. It prohibits "Installation of the Licensed Materials on any Computer other than a Computer owned or leased, and controlled, by Licensee, unless otherwise authorized in writing by Autodesk" - this is bad for portable use, and would technically mean every user would need to get written permission from Autodesk just to use it at the library, or on their friend's computer.
3. It also limits use outside of the country where you initially installed it - not a problem for most, but it is for some.
4. Lastly, while it doesn't explicitly say that the software is locked to a given PC, it infers that there are security measures within the software that you aren't allowed to bypass which are meant to stop the program being moved from one PC to another. If this is the case then it would be technically improbable that we could do it, let alone that it would be against the terms of the license.

Another observation: it is BIG. At 1.5+Gb it is somewhere between 150% and 300% the size of an install of LibreOffice Portable (depending on languages installed etc.)

With all of that working against us, I don't really think it is feasible as a PortableApp, sorry to say.

Following up on the above - A quick Google search tells me you can likely use Blender successfully as a replacement, which we conveniently have available here: https://portableapps.com/apps/graphics_pictures/blender_portable