Yes installing this with cable ties is an option, however this post box does not have a rear access facility. For installation to vertical railings I can recommend the Black Railings Post Box.

This unit has a smaller loading slot which may prevent hands from being inserted. However you may want to consider our Brabantia - B410 Stainless Steel Post Box which is fitted with security slats to prevent any mail from being stolen.

This post box isn't "self closing" All being well your postman will be considerate enough to ensure its fully closed once he has dispensed your mail.

This item is designed to me installed to a solid surface. Providing you can position the fixing holes so that the screws can be affixed to the gate securely this should be possible.