I had to cut some boxes in 3/8" T1-11 and I grabbed my biscuit jointer. It worked surprisingly well.
I did my lay out lines on 1 foot strips of blue painter tape in a tic tac toe pattern so they extended beyond the plate of the jointer and used the layout lines on the jointer to line up the blade. I did need to clean up the corners with a keyhole saw but the bulk of the cut was done and the hole was squared up.
Just be sure you hold the jointer tight so it doesn't walk.

I used one of those oscillating multi tools last time. Before that, was a drill & sabre saw.

I liked the jointer because you punch the hole exactly where you want it. Those layout lines and the cut marks on the jointer plate make it very precise. You still need to eat out the corners with the keyhole saw.

Greg,
That blue tape (from 3M I think), has to be one of the best things for setting stuff out, no matter what you are doing.
A friend of mine is in an electric wheelchair, it failed near the local railway tracks last month and required 5 people to get her away from the train that was coming, in the next 30 minutes.
What had happened was, there was an alloy block that holds the motor to the body of the wheel-chair, it had worn out over time and a corner had snapped off of the block.
I had a block of stainless steel here I was going to use for something else, but it sat here for ages.
I called on my mate who has a milling machine, marked the thing out with the blue tape, back in the day we would have used "Engineers Blue", a liquid solution that used to stain steel blue during fabrication and you could "bruise" the steel to reveal lines and punch points.

Long story short, it's been a while since I've used a milling machine, and I wrecked the first bit I made, second one was drilled, tapped and fitted right into place.

I use a plunge cutter or also called a osculating tool. I bought it last summer, and its my new favorite tool. No drilling, no corner clean ups, it just saws right straight in with on walking. Makes a very precise and clean cut. Also works well for cutting into cedar shingles, without ripping and tearing the shingles you want to leave. Just LOVE that tool.

Yup, oscillating cutters are great for thin pieces of wood and for drywall! Cutting through 3/4" hardwood is possible but you'd better not try making a 3-foot cut.

Those tools are commonly known as multi-tools, the most famous model is the Fein Multimaster. I've got one from Aldi's that quit working when I tried cutting a longer stretch of flooring but mysteriously worked again a few weeks later. In the meantime we bought a Bosch unit, which seems to be sturdier than the cheap one (and it better had to be for five times the price).

TR...

The Fein unit has an internal thermal cut-out that automatically re-sets when it gets too toasty.

It's nice to know that yours worked correctly.

Because these tools don't have a rotary blower/fan, they are really designed for intermittent use -- sort of like welders.

I had no issues with overheating when cutting 3/4 oak flooring. Both cross grain (end cuts) and with grain cuts. It was not 'quick' & I did not push the tool.

The blade condition is a factor -- as is the gumminess of the wood.

My experience has swung all over.

BTW, the Fein is a dream machine for flooring, which is outside our usual scope of activity.

The blades -- labeled as metal cutting + wood are NOT able to tolerate hardened metals -- even drywall screws. Been there, done that.

It's essential to keep the blades clean of any resin. They are much more vulnerable to tacky wood than any other.

I'm going to experiment with a Dremel to see if it can re-sharpen the 'E' blades.

The attachment core is hardened brittle steel. So it's wise policy to not stress the cutting too much. You'll tear out the pivot core... sort of like losing teeth on a starter gear.

The one that failed wasn't a Fein - it was a 29.99 Euro Aldi unit. I did think of a thermal cutout but when it didn't work after 24 hours (maybe even 48 hours) I just considered it dead and bought a Bosch. Weeks later I tried the cheapo again just for fun and it worked again.

Tesla:
I neglected to include that I used at least six (6) blades. The half moon style, and the basic wood straight.

Multiple passes, about 1/3 depth each worked good and seemed to keep the heat in check. One blade met the maker on a floor nail

BTW, this was a DIY project at my home.

I tried it on tile as well (with a carbide blade of course) because some expert had partly tiled over the boxes on one job but that didn't work too well. A traditional carbide file was much faster and I used tile nippers whenever I could. Metal blades would be great for cutting through nails but they don't seem to be available. The carbide blades are advertised for metal but only good for copper and other soft metals.