Hi NormK,
The formula to find the force needed at fully close.
F(kg) = (Mower Weight + (Platform Weight/2)) / (Tan Angle x 9.8)
Assuming Mower = 25kg, Platform = 15kg, Arm Angle = 5 degrees, Gravity = 9.8 m/s
F = (25 + (15/2)) / (Tan 5 x 9.8)
F = 38kg
Find a cross member that can withstand a 38kg of horizontal force of the scissor jack in the the middle of let's say 60cm long of cross member. It shouldn't bend.
Cheers,
TheCarbyMaster