Shifter for quick single speed to geared conversions

I have a single speed I've been using for single track. There are some group rides on gravel that I would like to participate in but the gearing is just too slow to be of use keeping up with a group on gravel. Right now I use my trail bike but that's just sort of dumb overkill for gravel riding. My single speed frame is all externally routed so I'm contemplating setting up a shifter and derailler on a single length of shifter housing I can pop on and off in 60 seconds, and remove the rear wheel and pop off the cog and pop on a cassette. I figure I can convert it back and forth in less than 5 minutes. I don't know why but I've been thinking I might like using a thumb shifter. If the indexing goes to crap I can always friction shift it. Are there any thumb shifters on the market past or present that have a hinged clamp I don't have to remove all my controls to install? I think a Paul's Thumbie mount would work with a bar end shifter. I don't know enough about modern 10-11 speed road groups to know what the compatibility is between the road stuff (bar end shifter) and mountain derailleurs and cassettes. I'm not too hung up on the number of gears I need, anything more than 8 that fits on a hyperglide hub would be sufficient. Are there any botique/upscale thumb shifters on the market I don't know about? Or old products that would fit the bill and work well? I haven't used or seen a thumb shifter since the early 90's so hopefully you guys can point me in the right direction. Although a hinged clamp would be Ideal I can abandon that feature if the best thumb options would preclude that feature.