I inherited 2 '60's Rocker covers with the tops cut out...........works great for the "running and warmed-up" adjustment.
Remember to only tighten the rocker arm 1/8th turn at a time............and give the lifter 10 sec. or so to "catch-up" to your adjustments. If you go too fast, you will feel the engine slow down, as you are "loading" the components.......
On an older engine with high miles, don't sweat it as much..............
With solid lifters, you can do it running.........but you would have to ask my old neighbor the trick for that..........he was doing it in the '50's on 265's and early 283's...........Duntov "SR" grind..........
