We use Gemini track. 99% of the time we apply to acylic (plexi glass) with Plastic Weld #3. Other than that pop rivet every 6" or so. The problem of using aluminum is the plastic track is going to expand in the heat and the aluminum will not, thus the track will buckle out and the letters could fall through. If you try to use aluminum slot the holes in the track and bolt on, not super tight. Use a lock nut and DO NOT over tighten, this way the track can move. Better yet weld it on some plex like it is designed for.
Polycarbonate track is a better choice because of the different rates of expansion. Polycarbonate is less likely to crack under the stress, you will want to use lots of rivets so the letters do not fall through the gaps that are created when the temperature gets high.