You can buy a generic mount that's bulky and flimsy or you can make your own in 15 minutes that will fit your phone perfectly. Regardless of the shape and size of your cellphone this method will fit.

Picture above is Samsung Galaxy that is approximately 5 inches long and 3 3/4 inches wide with an extended battery. Wider phones that are between 4 to 5 inches can use 1 inch schedule 40 PVC instead.

Things you will need:

3/4 inch schedule 40 PVC pipe (10 foot from local hardware store cost about $2.50)
Flashlight bicycle mount
Handsaw
Heat gun or stove top
File
2 AAA batteries used

Step 1: Measure and Cut PVC

Measure the length of your cellphone then cut off a section of PVC pipe about 2 inches longer than your cellphone. For example, if you cellphone measures 5 inches long then cut your PVC 7 inches long.

Step 2: Heat PVC

Heat the PVC for about 20-30 or until it's soft (just soft enough to flatten). Rotate PVC as you go about it to heat evenly.

Step 3: Flatten PVC

Place the PVC on flat surface between 2 AAA batteries and place another flat object on top (2x4 lumber works very well). Apply pressure for about 2 minutes or PVC has cooled off and becomes rigid. The batteries will prevent PVC from being too flat and also act as stabilizer.

Step 4: Mark Face

Place your phone over the PVC lengthwise and center it so that you have equal distance between top and bottom. Mark on PVC with pen at bottom edge of phone then remove phone.

Measure about 1 inch above marking and draw horizontal line. Draw another horizontal line about 2 inches above previous line. Now you have 2 horizontal lines 2 inches apart. Be sure this area does not overlap any buttons on your phone.

Draw a vertical line down the middle of PVC that bisect the 2 horizontal lines.

Draw these lines only on one side.

Step 5: Mark Edges

Draw vertical lines along edge of PVC (on both sides) connecting from horizontal lines to ends of PVC.

Step 6: Cut Along Edge

With your handsaw cut down along the middle edge on both ends as indicated by the red lines.

Step 7: Cut Face

Again with handsaw cut along the red lines half way through starting with the ends. Remove the half ends.

Cut along middle vertical red line. Again only half way through.

Step 8: Shape PVC

Heat the PVC with heat gun until soft and spread it to form a cross. Flatten it on flat surface.

When it has cooled and safe to handle with your hands, use file to smooth out the edges and corners.

With heat gun, heat and shape short arms of the cross so that it snugly hold the sides of cellphone. You should heat the arms just enough to shape it.

Now heat one of the longer arms so that it is curved and will catch the cellphone. Repeat for remaining arm so that it grips the cellphone on top and bottom.

Use file and/or handsaw to remove excess PVC.

When you have the shape down and it securely holds the cellphone, glue or screw PVC to flashlight mount.

Attach to bicycle handle and you are done. Congrats on your custom fitted cellphone mount!