Masking tape from one side, over the mitre, to the other side.
That'll pinch it tight until the glue sets and you can finagle it however you need.
That is a slick little trick with the tape. I am going to give that a try.
I have put a wood triangle on the inside, then clamped it lightly with quick release F-clamps. It I were going to do a lot of them, I would cut notches in a squared triangle so the clamps would be easier to set.
But a strap clamp would work great and is a usefui tool to have around for all kinds of things.
Use caution if you use the tape trick.
We do tons of floaters, most of them very large.
About 50 in the last 2 weeks.
We often have to build strainers or use 1/2" gator strips to prop up the canvas boards, dibond mounts, plexi-mounts, epoxy coated mounts and stretched canvases because they like to have the art in the floater about 1/8" below the face and believe me, we know what we're doing.
The design company that feeds us a constant pile of canvases uses many, many floaters in their designs.
Their crate and freight company just came today to pick up 81 pieces going to California.
I use the tape trick sometimes and it can work well, but I've been framing for 51 years now.
If you aren't familiar with the finishes that can take tape and those that can't, you might be in for a bad surprise when you pull the tape off and some of the frame finish comes off with the tape.
You can still use the tape trick but put a piece of your brown paper over the face and side of the floater to protect the finish on the mouldings that don't like to have tape on them and just pull the tape over the top of the moulding over the brown paper and stick the tape on the backside off the moulding.
I sometimes use tape over the top and sometimes use a strap clamp if needed but most of the time the frames join quite well without issues or having to go to extra steps.