Here is something I built out of a 4x4, paint, post cap, plywood base and some string swings. I got it set up to hold 2 mandos and a guitar and the back side is open for my next purchase. You could make it shorter for all mandolins. I got tired of having to put mandolins in and out of cases and I only had one stand for the guitar and none for mandolin, I really didn't want to take up any more floor space so this worked out good.
Sorry the pics aren't the best.
For anyone wanting to make one, here is what it took/cost:
4"x4"x8' post $8.00
Post Cap (pick your style) $8.00
Paint (2 cans) I used a textured paint $10
String Swings 3 x $10/each (I had one already)
Plywood base (I used a scap piece) <$5
Trim around base (scap) <$5
Screws to attach base $1
Felt Pads on base bottom $3
Took only a few hours (maybe 2-3 at most and most of that was mitering the trim around the base).
Note: spend the money on the string swings, don't try to use tool holders from Home Depot, they can ruin the finish on your instrument from what I understand.