Given the fact that you already have a metal frame underneath, I would think you could get by with only the 3/4” you already have. But your idea of routing only 1/8” deep and then laminating the top with HB sounds like an excellent idea. Contact cement would be perfect IMO. The HB would add some thickness and gives you a smoother top.
Also, one other option, because again of your metal frame already, is you could still route your 3/4 top to accept the track, but add thickness below the existing 3/4” layer, but below the track. Perhaps gluing a strip of 1/2” or 3/4” plywood 3-4” wide along the entire length of track. That would give the track screws something more to bit into and hold on.
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Also, one other option, because again of your metal frame already, is you could still route your 3/4 top to accept the track, but add thickness below the existing 3/4” layer, but below the track. Perhaps gluing a strip of 1/2” or 3/4” plywood 3-4” wide along the entire length of track. That would give the track screws something more to bit into and hold on.
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