A little late to the party, but, I use a leather ez-pass holder I got off of Amazon. Heavy-duty and has a long/strong Velcro strap that wraps around bars.
A tidbit- In Va, if you ride without your tag, the system will also take a pic of your plate and match it in the system automagically. However, after the system does that auto-matching a dozen or so times, it will start to charge an additional $1 fee billed as a "V-Toll" fee. I took a screenshot of my own EzPass account below to show some of it. The toll bastards are worse than bridge trolls at extorting more money from you.
Every time you see "V-Toll", that's a run through an ez-pass lane where it didn't read my tag (sometimes I legitimately did not have it, but many I did). This means that transaction was flagged for an additional fee, which they nicely add up and summarily charge you for with that line that says "VTOLL fee". The lines that say "toll transactions" are where it read the tag correctly.
Also, there is a difference in transponders. A motorcycle transponder in VA is a code "136" transponder (car/truck is 72) and it has to be programmed to send that 136 code. It is entirely possible a truck/car transponder will in fact be read correctly by some of the toll scanners (and often they do), BUT, in talking to the folks who administer the EZpass system it's not guaranteed to always match correctly so you could be getting hit with the v-toll fee just because the class code of the transponder does not match. The little transponders all look the same, but they send different codes.