I made up two sets of "low" roofs - the kind that would go over a 3" span rather than a 6" span. I tried using the TLX wall slotting mechanism, and I tried with just a tab. Neither really feel satisfactory. The tabs alone slip out too easily, and the slot doesn't hold the truss in place well enough - it tends to peek out at the bottom.
I'm now thinking of trying something a little different. I'll move the slot into the middle of the flap on the truss. I'll also make it a lot longer - as long as I can. Then, I'll make the tabs on the roof actually be a little bit in from the edges. It'll be a little harder to assemble, but it shouldn't be THAT bad. If they still don't hold in well enough, I'll make the tabs with a little notch at the bottom, so they hook onto the truss just enough to keep them from popping out. I think that mechanism should work for all different roof sizes fairly consistently.
The roof pieces I've made so far also are too wide by four times the thickness of the cardstock, which is actually enough to make it unsatisfactory. The reason is because the tabs on the edge of the roof are still inside the double-layer of truss wall. I will be sure to measure more carefully. At least I'm smart enough now to just print draft on cheap cardstock rather than photo-print the roof pieces every time...
One slight drawback to the normal TLX roofs is the fact that there are some untextured parts on the inside of the roof and truss. I'm not sure it's worth the extra effort to cover that up. Doing so would mean a lot more complexity to the build, and typically the roofs are enclosed, and then totally removed. Even if the inside of the roof is exposed, it's not that big of a deal (to me).