Check
unirdnas build journal
doubling the ply is over-done. with 3/4" you will be more than fine for only 5G, even 1/2" will work but I haven't seen good quality at HD or Loews around here. Just check the AGA or Perfecto stands for 90+G. Check into pocket hole joints for strong and easy joints.
I used 3/4" cabinet grade ply for a 90G stand. Worked very well in my opinion.
Good luck!