Which tbolt clamps?
 

McMaster-Carr

300 or 316 series ?

I really don't want to spend $8 per clamp. I just don't know how a zinc coating holds up.

OR

T-Bolt Clamps - Columbia River Mandrel Bending

304 SS and supports a site vendor. I don't know if the nut and bolt are SS or zinc coated steel.

What are your thoughts???

$2.40 a clamp.

cxracing.com: 1.5", 2", 2.25", 2.5", 2.75", 3" and 3.5" Stainless Steel Clamp 12 Pieces Set, Choose from Any Size.

I've used them a bunch, but they require one thing: a $.05 washer behind each nut, as the square housing behind the nut is roughly 7/16", along with the nut, and your wrench will often slip over it and lock up.

Edit: I know you need aluminum for your block off plate but I'm pretty sure most Home Depots will carry something that will work for that. Even a piece of steel wouldn't be much heavier, although I believe they should carry a piece of aluminum thick/big enough for your use.

I checked home depot today. They only had very thin sheet metal. The only aluminum they had was a 2" x 30" x 1/8" and it was $10. I can make quite a few pieces out of a 12" x 12" piece,a dn 3/8" thick leaves plenty of strength for tapping.