How secure will be a DIY bike chain made from towing chain purchased at the local hardware store?

If it's easy to cut at the hardware store, it will be easy to cut on the street.

The hardware store people cut the chain with a hydraulic cutter, not bolt cutters. I was there to see them cut me a sample of the second-strongest chain they had. It took 5 pumps of the floor-mounted hydraulic cutter.

On the other hand, I don't know how a hydraulic cutter compares to a bolt cutter. I just know that the word hydraulic sounds cool .

Compared to what?

Anything that's made by one man can be cut by another man who has the necessary tool. Bicycle security is a game of one-up-man-ship with potential thieves that you will always ultimately lose. The art is in balancing the cost, weight and bulk of of your security system with the degree of risk posed by the value of your bike and where you park it. If you park in the same place all the time a big, bulky, cheap chain that you can leave in place, even if it's not the most difficult to cut, may deter thieves who are looking for an easier target.

A hardware store towing chain will likely be a proof coil (grade 30) chain. Not terribly strong on the scale of chains. A grade 70 transport chain will be a lot harder to cut.
Available where truckers supplies are sold.

+ a link formed from square stock is not as easily cut as round ones ..

if they have to use a a grinding disc to cut it then it may be hardened.