I was going to chime in with the "use some Grade 5 Bolts" recommendation, but someone beat me to it. Those machine screws you had would bend like a pretzel, or just snap. Also, +1 on the fender washers, and nylock nuts too. Once it's in place and torqued down, spray the exposed bolt underneath with some black rust proofing paint (like rustoleum or the like) to keep it from rusting/corroding. Looks good!
2015 XLR Hyperlite 30HFS5 (mods being performed regularly)
2009 Salem LA 292fkds (gone)
Nights- ('12)23 ('13)23 ('14)15 ('15)31 ('16)25
2014 Harley Davidson FLHX (XLR cargo)
2014 VW Jetta TDI (diesel sipper, camping support vehicle)
2011 Ram 2500 CC 4X4 CTD, B&W Companion (toy hauler hauler)