To thos who asked, I didn't use anything special for 7mm rope, just a belay/rappel plate that looks like a metal circle with two oval slots, and a large diameter tubing auto locking carabiner. I used the same set up to rappel thousands of feet of fixed line on mountaineering trips. Belay plates are out of fashion now, replaced with ATC and Reverso type devices.

Later, I used an original Black Diamond ATC with single strand 7mm cord a fat carabiner. That is the key item, a carabiner with thick tubing.

The new BD Guide device, my current choice, is rated to work with 7.7 mm - 11mm rope but will handle 7mm using a Petz Attache carabiner.