The best theory I've seen for the model's construction was put forth by Daren Dochterman over on The Replica Prop Forum (his most specific posts about the construction (link)
and images to support his idea (link)
). His thought is that it was made of "gaffer's foil" (the foil used on film sets to shape light or make improptu flags) wrapped blue lighting gel material. This would have allowed a small stage light to have been put inside it. Shaw also chimes in on the possible use of the Catspaw 3.5" metal Enterprise in the episode. Here's the full thread (link).
Looking at photos, you can see the foil-like texture and these odd strips that look like tape folded around the maw.
It's not a windsock dipped in concrete, that's pretty clear.
Off-topic, but you might want to know that last month, the RPF server and registration at GoDaddy.com were attacked by a Russian hacker who cloned the server and transferred control of "therpf.com" to himself. The real site is up and running at thereplicapropforums.com
, but the site you just linked to is a Russian clone. Current information from the folks at the real site suggests he stole it to hold the URL ransom and to gain the revenue from their Google Adsense impressions. In the wake of the takeover, some members reported their own websites were attacked from Russian IP addresses. So by linking to the old site, you're potentially helping a hacker earn money, and exposing yourself and whoever clicks through to any Trojans or malware he might have loaded the forums up with.
The guys at the real RPF are trying to contact as many people as possible to top them from visiting the fake site. If you have a Replica Prop Forums account, more information can be found here