Only the detector needs a tube if that. Put it behind an aperture. In the mid-80's at a Trenton Fest I saw some guys who made a scanner out on a dot-matrix printer. They mounted the light sensor in a pointed pen barrel and got recognizable images. The IR is just for illumination.
Depending on train speed you may be talking 1" long strips. You might try printing flat black stripes on silver reflective material and see if they have that on sticky-back sheets.
I'll have to dig out my old spectrum and thermal printer to get flat black on 'silver' paper. It won't be sticky-back, though
Joking aside, a thin glossy white card would probably be reflective enough. I may be able to fit up to about 2" long strips. Train speed will be at a crawl, the scaled down (1/76)equivalent of about 3 - 4MPH (walking pace).
If you know what a hump-shunting yard is, I'm trying to automate one with 4 sidings, to sort a jumble of 16 wagons into numerical order. On the first shunt, siding 1 will get wagons numbered 1,5,9 and 13, siding 2 will get wagons numbered 2,6,10 and 14, siding 3 will get wagons numbered 3,7,11 and 15 and siding 4 will get wagons numbered 4,8,12 and 16. The loco will then pick up all the wagons in sidings 1 to 4 and again hump-shunt them. This time, siding 1 will get wagons numbered 1 to 4, etc.,etc. The wagons will then be in numerical order when the loco picks them up for the second time, no matter what order they started in.
This will give you some idea of what I'm trying to acheive.