Your profile suggests that its for use on a Vespa GTS 250 so the mounting of any such device is likely to on the LHS mirror stem with a short arm mount with shim, so RAM-B-149ZA ..
not like some twonk who wanted it located around his knees so it wouldn't be stolen, completely ignoring that he was looking down at a screen and ignoring the road and conditions around him .
What's yer budget? I get ask to mount various devices on scoots, from a TT One or Garmin Nuvi at the entry level (based on low price) and our weather/rain is not considered.
You'll also find divided thoughts among the forums whether to choose a Garmin or a TomTom, that said its your money, so its your choice.
If its a TT Rider Ver1 then use the mount kit already mentioned and stay with the TT Rider 1 cradle, yes there were with Rider V1 cradles which has two releases of it. Egon from this site made a clip to secure it and replacement TT Rider Ver1 cradles can be sourced *new* from
www.buybits.com. The Touratech cradle frame may be a tad too large to fit on yer scoot and it still uses the TT Rider connector parts to form part the finished cradle.
TT Rider Ver2 has a mount in the box and the only change here would be to use RAM-B-231Z (has shim included) and anchor onto the scoot LHS mirror stem arm then use arm, square base plate and cradle in the box.