This holder is designed to fit neatly into the tray on the dashboard of VW T5 van. It accommodates iPhones 6 & 7 (with a slim case on) but you could add a little bit of padding for bare phones.
I printed it with no support in ABS with an enclosed printer with no problems. It had an 8mm brim to minimise warping off the bed.
Just print it out, remove the tray from the van (pops up from the back) and drill/screw in the holder from underneath.
Feel free to remix and adapt for other phones / holders if you wish, just send me pictures!