1. remove bonnet from vechicle
2. obtain some bathroom scales (don't take bonnet to the bathroom, take the bathroom scales down to the vechicle LOL)
3. aquire a small ,light , strong person.
4.weigh the person note the weight
5. give the person the bonnet ask them to mount the scales note the weight.
6. The approx weight will be the rusult of point 5 MINUS the result of point 4 ......................
If you dont have bathroom scales at home, you could go to a local public wc with one of those 20p (red) scales...
To give a more true weight of the bonnet the person could carry out the weighing naked
I think I've found you the 'CF Bonnet' you are interested in.... it might need a bit of fettling to get it to fit correctly. I think the standard item is fine but each to his own..........