Building legs, with injuries?

Hi all,

I work on the computer, so my legs over the years have become quite skinny.

I'd like to build them, but I'm travelling full time, so no gym, and I have some injuries.

Two fused vertebrae in my neck, and two near the base of my spine.

My knees are also susceptible to injury, with fluid build up behind one of them.

So with a combination of bad knees, back and neck, how can I build my legs?

Ideally I'd like to get them quite strong. I've used an incline leg press in the past and loved it, but as I'm travelling, can't get access to that kind of gear.

Any advice appreciated.