I would recommend farming it out, the added bonus is that you can print white. I've had good results from both vistaprint and signs.com. If your mounting them under the ramp choose inside-glass as the print surface. Vistaprints 3m material is much nicer to work with but you are limited to their specific sizes. The downside of the 3m material is they have micro air release channels, which makes getting small air bubbles out easy, but leaves anything more than 1/4 inch behind the clear part of the decal looking frosted. If you want optical clarity beyond the ramp I would go with the signs.com decals.

https://www.vistaprint.com/signs-posters/car-window-decals?xnid=TopNav_Car+Window+Decals_Car+Signs_Signage+and+Trade+Show+Displays&xnav=TopNav
https://www.signs.com/clear-window-decals/