I apologize in advance for my very 'rough' sketches:
Get 2 14' 2X4s, and 2 8' 2X4s (your choice pressure treated or not, depending on your preferences) run the 2 14 footers on edge under long sides of greenhouse (can use 12' if it is exactly 12' long and don't want anything extending much beyond) From here, cut the 8' to fit between the 12 or 14's. and screw together (again, on edge).
You can attach the greenhouse 1 of 2 ways. I'm assuming the tubing is about 1/2 in diameter. Drill 1/2" dia. (or whatever the dia. of your tubing is) holes about 1 - 1.5 in. deep spaced at intervals to match your greenhouse support hoops. insert ends of greenhouse hoops. On inside of framing (or outside if you wish to attach bottom of netting to these) screw in eye bolts, and use bunge or wire or rope to loop around cross support frame of greenhouse & hook/tie to eye bolt to keep greenhouse secure to base
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2nd option: make base just a hair wider than greenhouse, and use u shaped brackets (can be found in plumbing aisle for supporting copper pipes) and secure 'legs; of greenhouse to wooden base with the u bolts. This will be a bit more tedious to 'take down' each fall, but will be quite sturdy and it the base is close to the ground on inside of wooden frame, it will almost completely eliminate the gap caused by teh 4" or so extended legs o greenhouse.
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There is a 3rd option for base design. use either of the 2 above, but extend the runners about 6" in front of greenhouse & taper/round leading edge of 'runners', also slightly bevel lead-bottom edge of crossmember of base OR do what I have done...get ahold of some old skis and with carriage bolts, bolt them to the underside of the 2 runner (12') boards. They work reasonably well!!.
Just note that by extending the runners and/or putting skis on them, you won't be able to get the 2 greenhouses tight together unless you put the 2 'backends' adjacent.)