On Thu, 12 Sep 2013 12:57:11 -0700 (PDT), rangerssuck
wrote:

How about these:
http://tamperproof.com/categories/br...removable.html

Breakaway Nut/Non-Removable Security Nut Aluminum
Hex nut breaks away when nut is tightened leaving only the tamper-resistant conical portion. No special tool is required - uses standard wrench for installation.

Looks tall enough. Not crazy about the conical shape but could live with it if
it works... Argh, have to ask for a quote.

or these:
http://www.fastenersuperstore.com/bo...FWho7AodyhgA1g

Carriage Bolts have round heads with square necks under the head to prevent the bolt from spinning as the nut is applied. Also offered with ribbed neck.

Not tall enough. Hard to explain exactly what I'm trying to do but it needs to
be tall enough to leave a 'bump' on the inside of a thick walled plastic part
so sand can stick.
--
William