OK so I answered a couple of my own questions.
When you buy one of those gravel vacs for aquariums, the end of it is very sharp plastic, and you’d cut the hell out of yourself using it as is. Today, I was going to do the plastic dip, but thought I’d try something inspired by my screwing around with sunglasses in the past. I used to buy Rayban wayfarers? Kind of wrap around but not enough for me, so I’d run the bridge under really hot water and gently gently bend them a little more. When they’d cool, the shape would stick.
With the aquarium tube, I filled up the sink with hot hot water, let the tube sit in there a few minutes, then used pliers to gently bend the rim outward a tiny bit. Once I’d got it started, I worked around and around with the handle of a wooden spoon to create a 1/8” rim perpendicular to the tube (sort of like I remember the business end of my old Boston Pumpworks tube). Then I kept going, bending the edge slightly back so the part that’d be against my belly was rounded.
My original plan was to then dip it in plastic dip, but I decided to try it out first as it was, and it actually works great. No need to dip.
I got a converter to downsize the aquarium tubing down to what the automotive vac pump uses and that’s got a tiny bit of leakage, I think. Probably that white tape you use on plumbing will fix it.
Cheap and effective. Attached are a couple photos.