What laudable efforts you are making! I was going to suggest the AUG one, but see that you are already onto it. Perhaps you could use it (it is so very easy and will wear well) as an introduction to better ones after they get accustomed to it. Meanwhile, you could consider some of the less challenging ones in the GR. There are some there that a congregation could easily master and love within two months of patient use. It is absolutely joyous that you are pursuing this path.

If it doesn't seem feasible for you to tackle an easier one from the GR, you could write one yourself in modal style. You could ask Fr Columba to write some for you. Meanwhile begin using the AUG one as one to use, say, in Ordinary Time.

It is very encouraging for all that you are doing this.

I'd encourage you to start playing around with doing compositions, you might find that it's not as hard to adapt as you think.

Starting with the GR Alleluia,

1. Create a super-simplified version which just has one note per syllable. Which note seems to be the most important one to pick? How many beats should each syllable be? Does it seem to flow together as a musical phrase?

2. Now add just a little complexity to it. On a note you want to extend, change it into a torculus or porrectus. Put a little flourish on the last syllable of a phrase.

3. Pick one place where it seems most appropriate to put in a longer melisma, like on the final syllable or the last stressed syllable.

This was my approach when creating new alleluias because my schola couldn't handle the complicated stuff but I didn't want to bail entirely. They weren't magnificent compositions by any means, but it was a great learning process. And most of them were singable, although I did find a few that didn't work for the group.

The other thing I did was to be pretty consistent on the alleluia refrain; I think I used three for Lent/Advent/Ordinary. The composition was for the verse, which was sung just by the schola and not the congregation. So we had a chance to practice it before Mass.

Good examples and clever work, Jonathan.
This is exactly what we need when even the simplest GR alleluyas are not feasible. And, not to be overlooked is the one in the AUG.

There is no excuse (except preferred dumbness - which is a 'reason' and not an 'excuse') for people to be singing these tacky triple alleluyas (yes, yes, I know: they're in the GS and are 'approved' [is there anything that isn't if Americans scream loud enough?]), and they are used in the Divine Office... which is exactly where they belong!)

With examples such as those you have shared hear, anyone could be singing an alleluya that really belongs at mass.

Fr Columba is an excruciatingly fine composer of chant. It would be a godsend were he to provide some actual mass alleluyas with simple jubili that any congregation could master in two Sundays.

Carl's advice is really good, and should encourage those who have even a modicum of resident original musical thought.

This is not exactly what you are looking for, but this is something similar I've been working on.