For me, the flush trim plane gets the most use out of all my planes. While block planes certainly do get used for most projects, every time I do a glue up that has squeeze out, I pull out the flush trim plane and clean it up. They're also incredibly useful for cutting edge banding.

The nature of the flush trim plane means it can't dig into the wood.

Step 1: What You'll Need

Some rare earth magnets - two 10mm x 2mm is enough
Some hardwood
A blockplane blade. I'm using a Luban/Quangsheng blade, but something as common as a Stanley #60 will do just fine too. Nothing too wide.

Step 2: Prep Your Stock

You really only need one face flat, the squareness of the sides and top don't matter, but it can make it easier for shaping later on.

You'll want a blank thats longer than your plane blade, but not wider. Thickness really depends on what shape you want the grips.

If it's wider, rip it down.

Step 3: Abutments

Rather than just relying on magnets, to give the plane some strength when pushing the blade I'm going with some wooden abutments.

Trace the shape of your plane blade onto a thin strip of wood (3mm or so) the same width as your plane body, then cut that out using a bandsaw, jigsaw, or coping saw.

Bonus tip: if you've got a course-ish blade on your bandsaw, or a large opening in the throat plate, clamp a piece of scrap MDF onto the bandsaw table to make a perfect zero clearance plate.

Repeat this for the rounded rectangle section of the plane.

Clean up the abutments with sandpaper or a rasp to get to your line, then glue it on with the plane blade in place.

Step 4: Drill Holes for Magnets

Its easier to drill these before shaping - two holes (one in front of the rounded rectangle, one behind), at least as deep as your magnets thickness. You can safely go a little bit deeper, because you can fill that with epoxy. The magnets are more of a safety blanket than anything - they hold the blade up, but the glued abutments do the heavy lifting.

Step 5: Finishing Details

Shape the plane to what suits you - there really isn't any right or wrong shape. I drew a few lines on mine using french curves to something that looked right for me.

Apply a finish - I went with a water based polyurethane, but really anything will work here, BLO, polyurethanes, waxes.

Glue in the magnets using either thick CA glue, or 2 part epoxy