Unfortunately, it's $20 wherever I look for a decent beanbag mount. Ridiculous! Garmin sells disk mounts for their products and many reputable dealers include dash mounts now. But what if you need a dashboard mount for your old GPS? Well it's pretty easy and cheap to make one.
Smooth, flat plastic or something surface (if it's not absolutely smooth, your suction cup won't mount to it!)
I used a cd case. You can use a jar lid.
Double sided mounting tape
Step 1: Get the appropriate materials
Obviously you'll want to try the suction mount before using the plastic.
Your scissors need to be pretty strong.
Ditto with the adhesive tape.
Step 2: Make the disks
Mark your circles on the side you won't be suctioning (the rougher side of the plastic).
I used a cup that happened to be slightly larger than my suction cup base (I could barely stick that sucker in the cup) with tape around the edges in order to mark my circles with my marker. Richer people can use that thing that helps you draw circles (protractor??)
Cut out your circles with scissors. Perfectionists can also file down the rough edges of the plastic.
Step 3: Put the tape on your disk
Step 4: Anticipate the curves of your dashboard
Favor the back of your dashboard - more adhesive should stick there than anywhere else. When you attach the back of the disk to the dashboard, you may find the front of the disk pops up and away from the dash. Where it was appropriate, I stuck another layer of adhesive onto the front (should be near the very front) of the disk in order to balance it. Remember, if you have to make a choice between the front or the back contacting the dash, favor the back of the disk and balance the very front with a little bit of adhesive.
Do NOT bend the disk so that all the adhesive contacts the dash, because the suction cup needs a flat surface to suck on. That's where adding layers comes in.
Don't overdo the layering. In my picture the back half of the disk contacts the dashboard, while in the front you see that the disk lifts off the dashboard but is supported by the extra layer of adhesive.
Sorry to write so much about this, but it's the most important and difficult step...
Anyway, since you've made it fairly level, you can now remove the protector and stick your disk in the appropriate position. Press down fairly hard to make sure it gets stuck, but don't bend your disk too much to do this. If you did the layering part well, you're not going to need to bend it.
Step 5: Try it out
If the suction does not stick, then you've bent your disk too much.
If the suction does stick and the GPS falls to the front, either you didn't balance the front well with extra adhesive or you did it too much and now the back doesn't contact the dashboard enough. Also, if the disk just pops right off the adhesive, you need a smoother disk to contact it.
If everything works, congratulations!
Protect your new mount by keeping the disk out of the sun - if the adhesive melts you'll have a bad time getting the melted part off, plus your GPS will fall down.
NE-Phil writes that the disk is easily seen and can motivate thieves! So I suggest putting some papers on your dash to cover the disk after dismounting the suction cup.