I asked a good friend of mine who packs. He said that some Jock Straps work well, be he did recommend a harness. He said look around at the various types that are available, and if you don't plan on using it for hard packing then a few of the cheaper options might be better suited.

I have these harness's and at least for me they are extremely comfortable. I should stress that I mine are used mainly for strap on play.

Leather Strap Rider

Jolly Rider

and RodeoH briefs (sadly, Eden does not carry these)

He also said that packers are extremely comfortable and he has yet to have issues with walking, sitting or even dancing As long as the packer is secured in some manner it also shouldn't fall out. I know that my harnesses securely hold dildos just fine.

I hope this helps.

1) What if it falls out when I'm walking (when I'm not home)
That would be a big problem, but I have never had this happen as I use under-wear style harnesses whenever packing, such as the RodeoH. And they can't fall out of that.

2) What if I can't get a harness?
You can get a harness. Maybe not immediately, but even on Government Assistance you would be able to get one eventually. Or look up how to make your own online, or apply for a program. There are lots of programs offering binders to underpriveledged transmen and there may be some for harnesses as well.

3) What if it obstructs me being able to walk naturally?
It won't physically prevent you from walking naturally. You may make yourself walk in an unnatural way because you're not used to it, but it won't actually make you unable to walk normally.