Ok so here is what you need:
1 3" hole saw
Sheet of 3/4 plywood( I used this cause it is sturdy)
Straight edge
Marker or chalk line
Power saw
Nylon or other rope

Step 1:

Ok so put all your tubes side by side butted up against one another on a sheet of plywood, add two inches mark that spot and remove tubes cut off that length. Then measure that pice and cut a second one just like the first. Now you should have two identical pieces. Then put tubes back on one pice leaving about an inch overhang on both ends. Now take the second pice and lay on top. make a grid on top lining up center of tubes in each row ( you need to do length and width). Now that you have your grid use the hole saw to cut your 3" holes on the intersections. After your finished ( this is optional ) drill two holes on each end and put rope through top half only. Cut four 3/4 In spacers from scrap wood and glue to bottom sheet in corners( should be 1-2 in long). Now take top half and slide over tubes. This will hold them in place. Take screws and screw corners down. Load them up, rig the fuses and you now have a multi shot rack! I save my tubes every year and so far have built four fifteen shot racks! I only save plastic ( hdpe) or fiberglass tubes cardboard can weaken over time and give you a nasty explosion so you can use cardboard tubes but I consider them single use then pitch them. I mentioned the pre- loaded ones for our Canadian pyro friends where reloadable tubes are illegal . When done take out four screws remove top half then put tubes in a box or tote put wood on top and save for next year. Have fun and be safe!