The window stickers and labels are a good idea, I've mentioned this on another thread. Even if you don't do anything else, it might just be enough to make the odd opportunist move on without looking further, if you're lucky.
There are lots of security bolt types available, but even if the fixings for the plate are standard overlong hex bolts with nyloc locking nuts on the reverse, it's going to delay them sufficiently - even if they happen to have two spanners the right size. As culzean says, they don't often come equipped with a grinder either, so the plate should be reasonably effective with a bit of thought going into the installation.
Note that all you are trying to do is delay them sufficiently to increase the risk of them getting interrupted.
The rebar is a good cheap DIY solution, 3/8" concrete reinforcing bar is relatively cheap from builder's merchants or scrapyards. It depends how helpful the garage is as to whether they would be interested in going to the trouble of fabricating the bits and welding them up to the pipe. However, labour time costs money, and in your case, the prefabricated plate may be the better option.