In making loft beds for the kids I've needed to do a lot of sanding. Some of it, like the spindles for the guard railing, would be a good fit for a stationary sander, but I don't have one. I do have a belt sander, however, and if you poke around online you can see lots of DIY belt sander stands, but they all look like a lot of work. Here's what I did instead:

This is a pair of nails hammered into my porch. Weather permitting I like to do as much woodworking outside as possible to annoy the neighbors minimize dust indoors. The nails are just wide enough for the handle on the sander, and angled up to hold it in place:

It feels very secure: I don't think the sander could jump up off the nails (which would be pretty inconvenient: runaway trigger-locked belt sanders are scary). I used it for eighteen spindles, and it worked well!

(No guarantees about how well other belt sander designs fit nails.)