I to was interested in the same but haven't been able to find supply of any online that were made purposely for either the Etrac 11" or the CTX3030 11" coil.
As time has gone by I've learnt to use such coils without one.
I do however recall someone making such out of a drum head/ skin with plastic outer ring either for Etrac or maybe Explorer. There was talk around about making "filler" pieces that would go into the spaces of the coil, that would help with keeping out sticks and grass, so maybe look at those and possibly making them out of a light weight material like packing neoprene or other .
Plastic drum heads/skins are usually light weight and thin, made of a pretty tough clear plastic material and would go the distance in closing the coil from underneath at least.
If you can't get one that allows the coil to fit in with it's cover , maybe remove the stock cover 11" so that you get a reduction in the circumference and then maybe use a sparing amount of silicon adhesive (easy to remove if you ever need to) to keep the drum skin attached to the CTX3030 11" coil.
Here's an example of supply in the UK for drum skins:
http://www.gear4music.com/Drum_Head/12-inch.html
There's also a possibility of one these working for the CTX3030 11" coil.
Tesoro 11" Round Closed White Coil Cover
http://www.ebay.com/itm/NEW-Tesoro-11-Round-Closed-White-Coil-Cover-Scuff-Authorized-Tesoro-Dealer-/322107540230?hash=item4aff1b0306:g:KLIAAOSwxfNXNgWY
Minelab also have the covers for their Commander series coils that is a Minelab 11" White Round Coil Cover that might also suit. Paint it black maybe
All the best and kind regards
David Di