Just using 2 of the screws wouldn't be strong enough. It's pretty easy to make the brackets, though. I used only a drill, hacksaw and a file. No major power tools. I also used one of these brackets.

The one I used is thicker than the one in the pic but I couldn't find and exact pic. I cut off each side and used one of the holes in each piece for the front bolts. The other two holes on each bracket had to be drilled because of the spacing.
Take one of the 6mm bolts with you to a hardware store. Get 2 6mm bolts that are a little shorter than the stock ones and get 2 6mm bolts that are a little longer than the stock bolts. Get 4 nuts and 8 washers for the 4 new bolts.
Use the 2 shorter bolts, 2 nuts and 4 washers (top and bottom for each bolt) up front.
Use the 2 longer bolts with washers (top) in the middle. These will screw into the rear set of frame threads.
Use 2 stock bolts with the other 2 nuts and washers (bottom) for the rear.
This may not make sense reading it but when you take it apart and fab the brackets it'll make sense.