Depends to a degree on what type of bike you're planning to take. I have one of the near-ubiquitous Thule ProRides. That takes my gravel bike no problem, and took my old Trek 3500 mountain bike similarly well. My newer mountain bikes which have the more modern 'relaxed' geometry were a much less satisfactory fit, having a wheelbase which was extremely marginal for the length of the rail. That's important because part of the retention system is based on strapping the wheels securely to the rail.
The racks that attach the bike via the front fork are more likely to work in that case. You do then need to be able to stash the wheel(s) inside the car (preferably in a wheel bag, especially if you're talking about mountain/gravel bikes), unless you also have a separate front wheel rack on the roof.
When you say you want to make sure that the rails won't leave marks on the hire car, do you mean the rails of the bike rack(s), or the roof bars on which the bike rack(s) are usually fitted? I'd be wary of taking my own roof bars to fit on to a hire car, particularly if it was a model I wasn't familiar with, since the mounting hardware for the bars might not be compatible with the longitudinal roof rails on the car. For example, my roof bars are specific to my model of car, since they fit to bolts which screw in to pre-installed mounting holes on the roof bars - a bit like bottle cage mounts on a bike frame. I've tried one or two types of the 'universal' roof bars and they were 'universally' unsuitable - mainly in my case because the roof rails on my car are pretty chunky (not that I watch the programme, but apparently Top Gear once tested one by landing a helicopter on it).
Can the hire company not supply roof bars? If they can then you should be able to fit pretty much any roof-bar fitting bike rack to them using the T-screws. If the roof bars don't have slots for T-screws then you'd need to have the adaptor kit for the rack as well, to be able to mount it to un-slotted roof bars.
Is there no bike hire available in Strathyre?