Well it likely a field base medkit bigger than the ones the soldiers carry, but doesn't have as much as you would back at the base.
I think the best way to implement this would be a special medkit that can be picked up in the rescue zone that would work in one turn, but you can't bring outside the rescue zone.
Maybe this could be a research topic, something like field base and the result is something like a compact self-propelled cart which carries medical supplies that a soldier can push along without slowing him down or making him tired and fits on the already overly cramped drop ship (a 20 hour flight on that thing must really suck). Kind of like the one they made for firefighters on the show "Prototype This!" except it doesn't carry fire fighting equipment. Plasma weapons are very nasty and dealing with major plasma burns is going to need more than a few bandages and some painkillers.
This could also heal injured soldiers like a hospital on the plane, but only up to half their max health and a fair bit slower. It won't heal a lot but it would help when you have one guy with one foot in the grave. I think on plane ride they should be able to do something even if it isn't much.
The proposal could go something along the lines of "The kind of wounds we are seeing from plasma weapons is little more than our current medical field kits can handle, especially the more serious wounds that need more immediate attention (like when a soldier is down to the single digits). With your permission... design a small field surgery/first aid kit that the soldiers can also take with them if the drop ship can't land near the intended target."
