For some reason I can no longer get the screen covers for my Garmin GPSMAP 64s, so i took to scouring the internet for a new idea, and stumbled on an idea suggested by "hillclima" on the New Zealand Shooter forums using clear "Scotch" brand packing tape. (available here in New Zealand)

After a bit of trail and error i got it to work and am pleased with the results so far.

Step 1: What I Used

A roll of clear Scotch brand packing tape ( wider than my screen)
Scissors
Alcohol prep pads ( or a lint free cloth and rubbing alcohol)
Lint free cloth from a mobile phone screen kit
Stiff plastic card ( i used one that i had got from a mobile phone screen kit -but guess you could use a bank card too?)
Wound dressing pad
Scalpel (or razor blade)

Step 2: Step One: Pre Cleaning

Using the alcohol clean the screen of any containment's – dust, dirt, adhesives, finger prints, grease... ( I used a combination of alcohol prep pads and the wound dressing pad )

Once the screen is clean let the alcohol evaporate, and polish with the lint free cloth.

Step 3: Step Two: Pulling the Tape

When you pull this tape off the roll it kinda comes in “jerks” which leave lines across the tape. I found you could avoid this by pulling the tape in the opposite direction low down against the surface.

Place the GPS on a hard surface on its back, with screen up most.
Hold the tape as you would normally with the sticky side down and pull out about and 5 cm to hold.(diagram a)
Then reverse the direction of the tape back over it self so the sticky side is now upper most. (diagram b)
Take the end that you are holding down low enough so that the non sticky side of the tape just past the bend back is now touching the non sticky still on the roll, (diagram c) holding this angle pull the tape smoothly away from the bend back until you have a length 5 cm bigger than our device screen.
Holding the roll in one hand and the end of the drawn out section in the other, turn the whole lot over taking care not to twist the tape, so the sticky side is now down.
Place the end you are holding down on the device 2 cm below the screen, and place the roll a head of the device, maintaining the tension on the roll in order to keep the tape above the screen.(diagram d)

Step 4: Step Three: Apply

Lower the end you are holding so that the sticky part of the tape just in front of your hand touches down approx 2 cm below the edge of the screen, push it so it sticks.

Then using the plastic card, push down on the plastic over the screen and push along to expel the bubbles as you would for a phone screen, extend tape approx 2 cm past the edge of the screen and stick down. (diagram e)

( I held the gps with my knees and the roll with the other hand to maintain tension while applying the tape to the screen)

Step 5: Step 4: Trim

Take the scalpel (or razor blade) and trace the edge of the screen with enough pressure to cut the tape but not enough to cut the device case/ rubber - pushing down the tape around the screen as you go. (diagram f)

Done!

Apparently the tape will yellow over time and sustain "wound" but peels off with ease and cleans up really easily for another application.