1. I didn't think the makeshift weld would do the trick, but I'm happy to say that it is HOLDING fast. Great. to hold or to hold fast Here "fast" is entirely appropriate for this context. "to hold fast" contrasts nicely with a welding failure. 2. I didn't think the makeshift weld would do the trick, but I'm happy to say that it is HELD fast. This one is ok, but not so obvious, The "it" now must be the things being held. It is held fast. So "it" must be obvious from context.

Excellent! Thanks, Michael.

Sentence 1. You could leave off the final "fast", too. It would be better without it. And yes, it's a continuous tense usage. For sentence 2, if you were to use the present perfect, you would want "it has held", instead of using "is held", which is wrong. You could only really use this tense if the weld had a specific, time+limited job to do, which had just completed. This is not a likely scenario.