Sideswipe just doesn't feel like Sideswipe without a spoiler. I didn't feel like putting a lot of time or money into one, so I did the most basic mod possible - cut the Nonnef Wheeljack spoiler parts to fit.
(I use G2 Sideswipe as his own bot, in a non-aligned Cybertronian faction, so that's why he has a different insignia).

I cut the Nonnef Wheeljack spoiler to fit, but cut a bit too much on one side. I tried to built it back up with some nail polish but it ended up a bit messy.
I painted the underside silver, and the top and sides in clear red, to mimic his windshield. Plain black probably would have worked better, but I like the effect I achieved.
For robot mode I just rotate the spoiler "wings" down the back of his legs.