I've not yet mounted mine, but it does seem a bit stiff - sounds like frankie's tip might do the trick.
You slip the etrex in from the top, the lip to press is at the bottom.
Yeah, it's a stupid amount of money for a poxy bit of plastic - and particularly one that isn't easily demountable between bikes.
I suspect I may have to do some fettling on mine - I thought I could fix it to the boom of the recumbent just between the seat and headset, but in second thoughts it might be a bit close to my thighs. The other alternative is to faff around making up a bracket (probably ply initially, and finally perspex/acrylic) to allow me to mount it off the top of the headset - it's underseat steering so there's no bars at a useful level. And in the case I may be better screwing on the carabiner mount, and saving the bike mount for the mtb.
BTW - this thread might be of interest:
http://yacf.co.uk/forum/index.php?topic=13073.0 for making a more substantial alternative (or at least, wrt the old Etrex mount)
It's actually for the older Etrex, using a different RAM mount, but there is a RAM mount available for the new Etrex too -
http://www.ram-mount.com/NewProducts/garminetrexmounts/tabid/3927/Default.aspx but it's not yet available in the UK:
http://www.ram-mount.co.uk/gps-handheld-garmin-c-360_169_659.html(the part no's RAM-HOL-GA48U)
Bodging a RAM mount on the 'bent might be a better option for me, raised up another 8" the extra width of the mount makes no odds..