EXPERIMENTAL, LIKELY TO CHANGE
Okay here goes. This is my design for a printable flat top picatinny rail for the Nerf Rapidstrike.
Before you print this, please be aware this requires SIGNIFICANT SHELL CUTTING! This should be obvious as you're losing the handle, but I thought I'd make it clear anyway. Also, I recommend epoxy to attach it and some sculpting putty to clean up any joins.
Only hardware required is some standard stryfe style screws. The Rapidstrike ones are, sadly, too long for this.
I've released the rail in four parts for those of you with smaller print beds. Print each part with the middle lying flat on the bed to ensure a flat join when screwed together.
I recommend screwing the parts together before gluing/epoxying this part onto your Rapidstrike to ensure alignment.
I'd recommend supports, simply for the parts where the front and back join.